Units for Distance and Size in the Universe | Las Cumbres Observatory Space Book Units for Distance and Size in the Universe Astronomers use many of the same units of measurement as other scientists. They often use meters for length, kilograms for mass, and seconds for time. However, the distances and sizes in the universe can be so big, that astronomers have invented more units to describe distance. Astronomical Units: Distances in the solar system are often measured in astronomical units (abbreviated AU). An astronomical unit is the average distance between the Earth and the Sun: 1 AU = 1.496 × 108 km = 93 million miles Jupiter is about 5.2 AU from the Sun and Pluto is about 39.5 AU from the Sun. The distance from the Sun to the center of the Milky Way is approximately 1.7 × 109 AU. Light-Years: To measure the distances between stars, astronomers often use light-years (abbreviated ly).

A light-year is the distance that light travels in a vacuum in one year: 1 ly = 9.5 × 1012 km = 63,240 AU Proxima Centauri is the nearest star to Earth (other than the Sun) and is 4.2 light-years away. This means light from Proxima Centauri takes 4.2 years to travel to Earth. Parsecs: Many astronomers prefer to use parsecs (abbreviated pc) to measure distance to stars. This is because its definition is closely related to a method of measuring the distances between stars. A parsec is the distance at which 1 AU subtends and angle of 1 arcsec. 1 pc = 3.09 × 1013 km = 3.26 ly For even greater distances, astronomers use kiloparsecs and megaparsecs (abbreviated kpc and Mpc).

1 kiloparsec = 1 kpc = 1000 pc = 103 pc 1 megaparsec = 1 Mpc = 1,000,000 pc = 106 pc Powers of Ten: The distances and sizes of of the objects astronomers study vary from very small, including atoms and atomic nuclei, to very large including galaxies, clusters of galaxies and the size of the universe. To describe such a huge range, astronomers need a way to avoid confusing terms like "a billion trillion" and "a millionth". Astronomers use a system called powers-of-ten notation, which consolidates all of the zeros that you would normally find attached to very large or small numbers such as 1,000,000,000,000 or 0.0000000001. All of the zeros are put in an exponent, which is written as a superscript, and indicates how many zeros you would need to write out the long form of the number. So for example: 100 = 1 104 = 10,000 and so on.

In powers-of-ten notation, numbers are written as a figure between one and ten multiplied by a power of ten. So for example, the distance to the Moon of 384,000 km can be re-written as 3.84 × 105 km. Notice that 3.84 is between one and ten. The same number could accurately be rewritten as 38.4 × 104 or 0.384 × 106, but the preferred form is to have the first number be between one and ten. Very small numbers can also be written using powers-of-ten notation. The exponent is negative for numbers less than one and indicates dividing by that number of tens. So for example: 100 = 1 Measuring the Universe | IAU Measuring the Universe Measuring the Universe The IAU and astronomical units Scientists use units all the time. The concept of an internationally standardised system of units is one of the most fundamental in experimental science. Everyone uses familiar units such as kilograms, kilometres and seconds and they are indispensable in daily life.

Scientists may need more exotic units such as measures of current, frequency and other scientific quantities, but the principle is the same, without an agreed scheme of measurement, scientists could not share results and there could be disastrous and costly mistakes. The International Astronomical Union (IAU) is responsible for maintaining and approving a special set of units in astronomy, formally defined in 1976. One of the most important of these is the astronomical unit. It is a unit of length approximating the Sun-Earth distance (of about 150 million kilometres) which is of convenient use in astronomy. According to its definition adopted by the XXVIIIth General Asssembly of the IAU ( IAU 2012 Resolution B2 ), the astronomical unit is a conventional unit of length equal to 149 597 870 700 m exactly. This definition is valid irrespective of the used time scale. The unique symbol for the astronomical unit is au.

The IAU also defines other astronomical units: the astronomical unit of time is 1 day (d) of 86,400 SI seconds (s) (SI is the International System of Units) and the astronomical unit of mass is equal to the mass of the Sun, 1.9891×1030 kg. Beyond the Solar System the distances in astronomy are so great that using the au becomes too cumbersome. The IAU recognises several other distance units to be used on different scales. For studies of the structure of the Milky Way, our local galaxy, the parsec (pc) is the usual choice. This is equivalent to about 30.857×1012 km, or about 206,000 aus, and is itself defined in terms of the au – as the distance at which one Astronomical Unit subtends an angle of one arcsecond. Alternatively the light-year (ly) is sometimes used in scientific papers as a distance unit, although its use is mostly confined to popular publications and similar media.

The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU. To think of it in easily accessible terms, the light-year is 9,460,730,472,580.8 km or 63,241 au. While smaller than the parsec, it is still an incredibly large distance. Defining a unit is often more complex than first appears. For instance, to define a light-year it is necessary to understand exactly what a year is. When referring to a year in the precisely defined astronomical sense, it should be written with the indefinite article “a” as “a year”. Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. The IAU also recognises a Julian century of 36,525 days in the fundamental formulas for precession ( more info ).

Other measurements of time such as sidereal, solar and universal time are not suitable for measuring precise intervals of time, since the rate of rotation of Earth, on which they ultimately depend, is variable with respect to the second. Units Used in Astronomy to Measure Distances - In astronomy, the distances are so huge that it becomes awkward to measure in terms of miles and kilometers. Below are some of the units used to measure distances in astronomy. Astronomical Unit (AU) An Astronomical Unit (AU) is the average distance between the Earth and the Sun, and is used to measure distances within our solar system. 1 AU = 150 million kilometers or 93 million miles (approximately) Light-Year (ly) Beyond the solar system, even the Astronomical Unit becomes a relatively small distance. This is where the Light-Year (ly) comes into picture. It is the distance traveled by light in vacuum for a year. The speed of light is about 186,000 miles or 300,000 kilometers per second. This makes the distance of 1 light-year to be about 9.5 trillion kilometers or about 6 trillion miles.

1 ly = 9.5 trillion kilometers or 6 trillion miles (approximately) 1 ly = 63,241 AU (approximately) Light-years are usually used to measure distances between stars. The closest star to our Sun is Proxima Centauri and is at a distance of 4.2 light-years. Parsec (pc) In 1838, Friedrich Wilhelm Bessel used the method of triangulation and the parallax effect to calculate the distance of a star from our Sun. The unit to describe this distance from the Sun to the astronomical object was termed as the Parsec which stands for the “parallax of one arc second”. A Parsec is about 3.26 light-years. This is equal to just under 31 trillion kilometers or just over 19 trillion miles.

1 pc = 31 trillion kilometers or 19 trillion miles (approximately) 1 pc = 3.26 Light-Years (approximately) 1 pc = 206,264 Astronomical Units (approximately) In order to understand the meaning of Parsec, it is important to understand the following terms: Triangulation: Triangulation is an indirect method of measuring distances using geometry with the help of a “baseline” and two known angles from the baseline to the object whose distance is to be calculated. Note: Establishment of one angle as a 90° angle simplifies the calculation, but it is not necessary. Parallax: Hold your finger in front of you with reference to any background object. Now look at the finger with one eye, and then with the other. You will notice that there is a change in the position of the finger with respect to the background object. This apparent movement is called parallax. Source: Wikipedia Arc Second: If we consider one circle to be neatly divided into 360 parts, then each part is a degree. Each degree can be further divided into 60 equal parts, where one part is an arc minute.

When an arc minute is further divided into 60 equal parts, each part is equal to one arc second. Calculation of one Parsec: In order to measure the parallax effect of a star with respect to background stars, we would need a big enough baseline. For this we make the first observation of the star from the earth being on one side of the Sun, and the second observation six months later, when the earth is on the opposite side of the Sun in its orbit. This would establish a distance of 2 AU between the two positions from which the star is observed. Source: Wikipedia We can assume an imaginary right triangle in space with its three points being the distant star being observed, the earth and the Sun. We know the distance between the Sun and the Earth to be 1 AU and the angle at the Sun’s corner to be 90°. The angle at the star’s corner is the parallax angle. The distance between the Sun and the star can be found out using trigonometry. When the parallax angle at the star’s corner is 1 arc second, the distance between the Sun and the star is 1 parsec.

Using the above calculation, 1 parsec is approximately equal to 3.26 lys. Astronomical Scales | Free Lesson Plans | Teachers | Digital textbooks and standards-aligned educational resources Billions of light-years The farthest known galaxy (the edge of the observable universe) is 13 billion light-years away. 3. Explain that one way to put the enormous sizes and distances of space into perspective is to compare them to smaller scales that are easier to grasp. In this activity, students will convert distances and sizes in space to smaller units. To begin, distribute the Classroom Activity Sheet: Understanding Sizes and Distances in the Universe, and have students work in pairs to answer the questions. 4. To help students understand how to solve these problems, you may wish to do the following problem together as a class: Problem: Using a scale in which a quarter represents Earth, what would the distance from Earth to the moon be? Solution: Three pieces of information are needed in order to determine this scale distance to the moon: the diameter of the quarter, Earth’s diameter, and the actual distance from Earth to the moon. Measuring the quarter reveals that it has a diameter of 1 inch.

Earth’s diameter is about 8,000 miles. The actual distance from Earth to the moon is an average of 240,000 miles, although this distance can vary with the moon’s orbit around Earth. For these calculations, though, the average can be used. Now that we have these three pieces of information, we can find the fourth piece (the scale distanced) by setting up the following ratio: This is equivalent to the statement “The diameter of a quarter is to Earth’s diameter as our scale distance is to the actual average Earth-moon distance.” Substituting what we know shows us that: Remember that it’s important to keep track of the units. If Earth’s diameter had been given in kilometers, it would be incorrect to use 240,000 miles for the Earth-moon distance. We would need to convert that distance to kilometers, too. Because both diameters are given in miles, they cancel each other and can be crossed out of the equation. In this problem, we should expect our result to be in inches, the same unit as the quarter’s diameter.

By multiplying both sides of the equation by 240,000 miles to isolated, we find that d = (240,000 miles) × (1 inch/8,000 miles) = 30 inches So, at this scale, the distance between Earth and the moon would be 30 inches. 5. Before students start working on the problems, it may be useful to go over scientific notation, which is a helpful way to deal with large numbers. Use the following examples to illustrate the powers of 10: 1 can be written as 100(because anything to the power zero is 1). 10 can be written as 101(because anything to the first power is itself). 100 can be written as 102(because 10 multiplied by itself, or 10 × 10, equals 100). 1,000 can be written as 103(because 10 multiplied three times, or 10 × 10 × 10, equals 1,000).

Explain that we can use these powers of 10 to represent decimal places, too: 3.4 can be written as 3.4 × 100. 99.1 can be written as 9.9 × 101. 4,526 can be written as 4.526 × 103. Review the properties of exponents to make scientific notation even more useful: When multiplying two numbers with exponents, if the base numbers are the same, just add the exponents. For example, 105× 103= 108. When dividing two numbers with exponents, if the base numbers are the same, subtract the exponents. For example, 104/102= 102. 6. Have each pair of students solve the problems listed below, which also appear on the Classroom Activity Sheet: Understanding Sizes and Distances in the Universe. Also included for students are constants that provide helpful information to be used in scaling. Students must figure out which information is needed to solve each problem. Students can work with partners to solve the problems, but each student should fill out his or her own sheet.

All the questions from the Classroom Activity Sheet and the answers are listed below. Questions on the Classroom Activity Sheet: Understanding Sizes and Distances in the Universe If Earth were the size of a penny how large would the sun be?(81 inches, or 6.7 feet, in diameter) how far away would the sun be?(8718.75 inches, 726.5 feet, 242 yards) What is located about that distance from your classroom?(Answers will vary.) If the sun were the size of a basketball how far away would Neptune be from the sun?(3237 feet, or 0.6 miles) how far away would the nearest star, Proxima Centauri, be from the sun?(5,538 miles) Find two places on a world map that are about this distance apart.(Answers will vary.) how far would it be to the center of the Milky Way?

(36,538,218 miles) About how many trips to the moon does this distance equal?(152) If the Milky Way were the size of a football field how far away would the Andromeda galaxy be?(6,600 feet, or 1.25 miles) how far would it be to the farthest known galaxy?(39 million feet, or 7,386 miles) Find two places on a world map that are about this distance apart.(Answers will vary.) Helpful Measurements A penny is about ¾ inch in diameter. Earth is 8,000 miles across. The sun has a diameter of 861,000 miles. One mile equals 5,280 feet. The average distance from Earth to the sun is 93 million miles. A basketball is roughly 12 inches in diameter. Neptune is 30 AU from the sun, or 2.79 billion miles. One light-year is 6 trillion miles. The nearest star, Proxima Centauri, is 4.2 light-years away.

The sun’s distance from the center of the Milky Way is about 30,000 light-years. A football field is 100 yards (300 feet) long. The Milky Way is about 100,000 light-years across. The distance to the Andromeda galaxy is 2.2 × 106light-years. The farthest known galaxy is 13 billion light-years away. 7. Assign the Take-Home Activity Sheet: Comparing the Sizes of Planets for homework. If time permits, discuss the answers in class. You could have students draw the planets to scale to compare the sizes of different planets visually. How to Measure Distances in the Universe - AstroEd AstroEd How to Measure Distances in the Universe From AstroEd 8 Redshifts Lasers and Radio Signal Timing Distances to the Moon are measured by timing a pulse of laser light reflected from mirrors placed on the surface by Apollo astronauts in the 1960's and 1970's. A laser pulse travels 400,000 kilometers from the Earth to the Moon in 1.3 seconds.

Send the pulse out, and 2.6 second later it comes back to you. McDonald Observatory sending a laser beam to bounce back from mirrors on the Moon Distances to Sun and planets are measured by timing radio signals from man-made satellites orbiting around those planets. The images of Saturn returned by the Cassini satellite take about 1 hour and 23 minutes to travel 1,300,000,000 kilometers to reach us. Within the solar system we measure distances by comparison to the Earth-Sun distance, and use the Astronomical Unit as the measuring stick. Since the Earth's orbit is an ellipse, astronomers prefer to define the "AU" for an imaginary planet that would orbit in the Sun in a circle in the same time it takes for Earth to orbit in its ellipse. For most purposes this means that the AU is the average distance from the Earth to the Sun. It is precisely 1 Astronomical Unit = 149,597,871.5 kilometers or about 150 million km, 93 million miles, 8.3 light minutes. Triangulation Triangulation has been used to measure distances for thousands of years.

Greek astronomers used it to try to determine distances of the Moon and Sun. Kepler used it to find distances to the planets. Surveyors use it to locate to locate property boundaries and plan construction. You could use it for something as simple as finding the height of a telephone pole without climbing it, tape measure in hand. A simple example in astronomy occurs when we measure the direction to an object seen when the Earth is first on one side of its orbit, and then again six months later when Earth is on the other side. Triangulation: The larger the angle AC and the smaller the angle BC, the more distant the object It's easy to see from this figure that the size of the angles tells us the distance B since we know that A, one astronomical unit, is 150,000,000 km. Let's call the angle to the object AC, and agree to measure it from the direction to the Sun. The triangle with the sides A, B, and C is a "right" triangle. That's one that has a "right" or 90 degree angle. The right angle is marked in the figure; it's the one that is at the Sun.

In a right triangle, the tangent of an angle is the ratio of the side opposite the angle to the side next to the angle. In this case, we'd say Tangent of the angle AC = B/A If you know the angle, and can calculate its tangent, then the distance B is simply B = A x Tangent of the angle AC In the solar system "A" is the distance from Earth to the Sun, the astronomical unit, 150,000,000 km or 93,000,000 miles. Suppose you see a mystery object like this one, and you want to learn where and what it is. You can see how big it appears to be, but since you do not know its distance, you cannot tell how big it really is. Mystery Object Comet Holmes in the Kuiper Belt You measure angle AC to be 89 degrees. Type 89 into your calculator and press tan to find that the tangent of angle 89 degrees is 57.3.

"A" in our equation is just 1 astronomical unit: Distance = A x tan(89) Distance = 1 x 57.3 astronomical units The greater the angle, the more distant the object. This mystery object is 57.3 AU away, somewhere in the vicinity of the Kuiper Belt. Would you rather know how far it is in kilometers? The astronomical unit "A" is 150,000,000 km, and our object is at Distance = 150,000,000 x tan(89o) kiolmeters Distance = 150,000,000 x 57.3 By the way, since light travels 300,000 kilometers per second, it takes 8,593,494,245 /300,000 = 28,644 seconds to reach us. That's nearly 8 hours! Parallax Parallax is the same as triangulation. We measure how much the direction to a distant object changes when it is observed from opposite sides of Earth's orbit.

Distant stars provide a background for measuring the closer ones. When out on an evening's stroll, and a nearby tree seems to go by quickly as we walk, while the distant Moon seems to stand still overhead, we're experiencing parallax. When Earth moves from one side of the Sun to the other, a nearby star will appear to shift against the background of distant stars. The greater the shift, the closer the star. Parallax measures the shift in the angle between a given star and very distant ones to obtain the small angle of triangulation Since the sky is a great hemisphere, we can think of it in degrees, the horizon being 0 and overhead being 90 degrees up. Each little degree may be divided into 60 tiny arcminutes. Each arcminute may be divided into 60 even smaller arcseconds. If a star shifts one arcsecond when the Earth moves one astronomical unit through space, its angle of parallax is one arcsecond, and the star is one parsec distant, about 3.26 light years away. One second of arc is a very small angle. Imagine a quarter held up in front of you at arm's length. That's about 1 degree across.

Now take that quarter 4 kilometers or about 2.5 miles away. It's one arcsecond across. A parallax of 1 second of arc indicates a star is 1 parsec (pc) distant, about 3.26 light years (ly) Example: Suppose we see a small red star and we want to know how distant it is. Mystery Star This time we use angle BC in the figure. The parallax that we measure from Earth is the same as the angle one astronomical unit would be if seen from the star. Just as before, the tangent is the ratio of the side opposite the angle to the side near it in this right triangle: tangent of the angle BC = A/B B = A / tan(BC) The angles we measure for stars are very, very small. That makes the math very simple, because for a very small angle in arcseconds tan(BC) = BC/206265 B = A x 206265 / BC It's easy now. A is one astronomical unit, the distance from Earth to the Sun. If BC is one arcsecond, the star is 1 x 206,265 astronomical units away.

That's one parsec, 3.26 light years. The smaller the parallax, the greater the distance: Distance in parsecs = 1 / Parallax in arcseconds What if this small red mystery star had a parallax of 0.75 arcseconds? It's distance would be Distance = 1/ 0.75 = 1.33 parsecs Distance =1.33 x 3.26 = 4.3 light years The only star this nearby is Proxima Centauri, the nearest star to us beyond the Sun. It's visible only with a good telescope from the southern hemisphere, a non-descript red dot among hundreds of thousands of other stars in the sky. When we see this star at night, we are seeing it as it was 4.3 years ago because the light took that long to reach us. It is set apart from all the others by having the largest parallax of any star. Because it is so close, its motion relative to the Sun in our galaxy carries it past us at an unusually fast clip.

Year to year it moves across the sky with the combined back and forth motion of its parallax, and the steady drift of its proper motion through the galaxy. Proxima Centauri, the nearest star to Earth, about 4.3 light years away Inverse Square Once the brightness of a specific type of star is known close up, we can tell the distance of similar stars by how much fainter they appear when farther away. The rule that applies is the Inverse Square Law. If at a certain distance the brightness of a star is known, then at twice the distance it will only be 1/4th as bright in a telescope. At three times the distance the brightness will be only 1/9th. Conversely, if a star of known brightness appears 1/4th as bright as one at a known distance, the star must be twice as far away. If a star appears 1/9th as bright, it must be 3 times the distance of the original star. Your eye senses this as "brightness". An astronomer counts the number of photons of light from the star her telescope captures each second.

Inverse square law The more distant a star, the more area its light is spread over when it arrives at our telescope, and the fainter it appears to be. An Example Suppose we see a Type Ia supernova in a galaxy 1,000,000 ly away. Then we see another Type Ia supernova in another galaxy, only this time it appears only 1/4th as bright as the first one. How far is the second galaxy? The inverse square law says that the intensity falls as the square of the distance. For the second galaxy compared to the first: Intensity = 1 / (Distance)2 Distance = 2 2 x 1,000,000 ly = 2,000,000 ly The second galaxy is twice as distant as the first. It would be 2,000,000 ly away. More Examples Suppose the Type Ia supernova were 1/9th as bright. It would be 3 times more distant. If the supernova were 1/16th as bright it would be 4 times more distant.

If the Type Ia supernova were 1/10,000th as bright it would be 100 times more distant, 100,000,000 ly away. If the Type Ia supernova were 1/127690000th as bright it would be 11300 times more distant! If it were 11,300 times more distant than 1,000,000 ly away, it would be 11,300,000,000 ly years away ...11.3 billion light years away !!! SN 1997 The oldest and most distant Type Ia supernova ever seen is SN 1997 11.3 billion ly away !!! Distance to the Magellanic Clouds Once we knew the brightness and distance for different types of stars within our Milky Way galaxy, and we had ways to identify the type of star they were, we could measure the distance to the nearby Magellanic Clouds using the inverse square method: Identify a star of the same type as a nearby one at a known disance. Compare their brightnesses.

Calculate the distance to the Magellanic Cloud using the Inverse Square Law. The normal stars that were found in the Magellanic Clouds were too faint, however, to be used in galaxies at even greater distances. Cepheid Variables Over 80 years ago, Henrietta Leavitt worked at Harvard College Observatory measuring photographs of Cepheid variable stars in the Small Magellanic Cloud, a compact galaxy that is a nearby companion to our Milky Way. These stars are similar to one that is relatively nearby, and that appears in the constellation of Cepheus. They are variable, increasing, decreasing, and increasing in brightness again. Some of them may go through a cycle in less than a day. Others may take weeks. Henreitta Leavitt was trying to make sense of the variations. Henreitta Leavitt and the Small Magellanic Cloud She found that the Cepheid's actual brightness could be predicted by their period, the time it takes for them to go through a cycle of change. This is called the Period-Luminosity Relationship. It's a seemingly mundane detail of the properties of these aging, pulsing, stars. Yet, it holds a key to finding the distances to galaxies.

These stars are so bright, and their variations so distinctive, that we can pick them out of the crowd. The period of their variation tells us how bright they really are. It is just like reading the wattage on a lightbulb. For instance, those blinking once every 3 days were about 800 times the brightness of the Sun, whereas those blinking slower, once in 30 days, were 10,000 times the Sun's brightness. With their brightnesses known, distances to galaxies in which they resided could be determined by using the inverse square method. Edwin Hubble found Cepheid variables in the Andromeda Galaxy, which place it 2.5 million light years away. It's the most distant object you can see in the sky without a telescope. The light that made this image left the galaxy 2.5 million years ago. Hubble's original photograph on which he first identified a Cepheid variable (VAR!) in the Andromeda Galaxy The same Cepheid variable is faint in the right image and much brighter in the left image. Can you find it?

(Hint: It is near the top of the images.) Because of photography, and the high average brightness of Cepheid variable stars, distances to galaxies as far as 60 million light years away (such as those in the Virgo Cluster of galaxies shown below) could be measured. Virgo Cluster of galaxies, about 60 million light years away Type Ia Supernovae Small stars, those without enough mass to cause a supernova explosion, become white dwarfs. Occasionally a white dwarf will gather mass from a nearby companion star, reach the Chandrasekhar Limit of 1.4 solar masses, explode, and be seen as a Type Ia supernova. Since all Type Ia Supernova have the same mass, they have the same brightness. If one appears dimmer, we know it is more distant. We need only apply the Inverse Square Law and we will know the distance to the galaxy in which it appears! Type Ia Supernova 1994D in galaxy NGC 4526 Even brighter than Cepheid variables, Type Ia Supernovae are as bright as a whole galaxy. They have been used to measure distances to galaxies as far as 11 billion light years away.

Flash of a Type Ia supernova in a distant galaxy Type Ia Supernova are brighter than surrounding stars in these images taken in visible, x-ray, and infrared light Redshifts Train whistles sound lower pitched as they recede, due to the moving source of the sound waves. This is called the Doppler shift. Light waves from a galaxy that is moving away also appear longer in wavelength, lower in frequency, and therefore redder in color. This effect is called the redshift, and it provides a way for us to measure distances to galaxies. In Stephan's Quintet, a cluster of galaxies in Pegasus, it is easy to see that one galaxy is not moving away as quickly as the others. Stephan's Quintet galaxies receding faster appear slightly more red than the one that is not receding as fast Once Edwin Hubble had identified Cepheid variables in Andromeda galaxy, he could use the Period-Luminosity Relationship, discovered by Henrietta Levitt, to find the brightnesses of the stars. With that, and the inverse square law, he could measure the distance to the Andromeda galaxy, and to other galaxies in which Cepheid variables occurred. Hubble noticed a pattern in this new data.

He noticed the greater the distance of the galaxy, the greater its redshift. Almost all the galaxies he measured appeared to be moving away from us, and the greater the distance of the galaxy, the faster it was going. His original measurements have been replaced by others of more accuracy, and extended out much farther. The relationship he discovered is called the Hubble Law. Cepheid variables are not bright enough to use for distance measurements much beyond the 60 million light years of the Virgo Cluster. The Type Ia supernovae are so much brighter that we can see them billions of light years away. The Hubble Law holds, even at these distances; even looking back billions of years into the past, it was found that the distance of a galaxy is proportional to its redshift. Now the redshift could provide the distance to any galaxy. The more distant the galaxy the greater the redshift. Distance could be measured in redshifts. A cluster of galaxies with an average redshift z = 0.01 means the galaxies are moving away at about 0.01th the speed of light.

If it is has a redshift z = 0.02 it is moving away at twice that speed, and is twice as far away. Astronomers measure the wavelength of light by analyzing the spectrum of a star or even an entire galaxy. When we look at a distant receding galaxy we see all the wavelengths of its spectral features increased by the same factor, 1+z. When z is small it is approximately v/c where "v" is the speed of the galaxy moving away, and "c" is the speed of light. However, matter cannot travel travel faster than the speed of light, and at larger z the speeds just get closer and closer to "c". We will look at how this works in more detail in a later unit when we explore the effects of relativity. For now, remember that the most distant galaxies appear to recede from us at nearly the speed of light, and that the more distant they are, the more its spectral features are reddened, shifted to longer and longer wavelengths. Not only could distances be measured, but since all distant galaxies are receding from us, and the more distant a galaxy, the faster the galaxy it goes, it appears that the universe is expanding!

Not only that, but if you follow their paths backwards, it seems that all galaxies would at some time have been in the same place, beginning at a singular event that we call the Big Bang! By the end of next week you will be able easily to measure the size of the visible Universe, and to follow the of galaxies backwards in time to the beginning of the universe. In all of human history only a few of the greatest thinkers have been able to do this, but you will find it easy! The redshift can give the distances of galaxies even in the Hubble Ultra Deep Field Can you tell which galaxies are more distant? (Note exceptions: elliptical galaxies are redder by nature, and galaxies come in a range of sizes and types.) Measuring the age and size of the Universe | ESA/Hubble | ESA/Hubble Couldn't save page :( Please try again. Measuring the age and size of the Universe The search for Cepheids Spiral galaxy NGC 4603 containing Cepheids being used for distance measurements. The top ranked scientific justification for building Hubble was to determine the size and age of the Universe through observations of Cepheid variables in distant galaxies.

This scientific goal was so important that it put constraints on the lower limit of the size of Hubble's primary mirror. Cepheids are a special type of variable star with very stable and predictable brightness variations. The period of these variations depends on physical properties of the stars such as their mass and true brightness. This means that astronomers, just by looking at the variability of their light, can find out about the Cepheids' physical nature, which then can be used very effectively to determine their distance. For this reason cosmologists call Cepheids 'standard candles'. Astronomers have used Hubble to observe Cepheids with extraordinary results. The Cepheids have then been used as stepping-stones to make distance measurements for supernovae, which have, in turn, given a measure for the scale of the Universe. Today we know the age of the Universe to a much higher precision than before Hubble: around 13.7 billion years. "We certainly live in exciting times. Hubble has made enormous progress possible within cosmology. Today we have a much more unified cosmological picture than was possible even five years ago when people were talking of ‘The Cosmology in Crisis’.

We have seen a dramatic change from misery to glory!" Gustav A. Tammann The expansion of the Universe Supernova 1994D (lower left). One of Hubble's initial 'core' purposes was to determine the rate of expansion of the Universe, known to astronomers as the "Hubble Constant". After eight years of Cepheid observations this work was concluded by finding that the expansion increases with 70 km/second for every 3.26 million light-years you look further out into space. Hubble's sharp vision means that it can see exploding stars, supernovae that are billions of light years away and difficult for other telescopes to study. A supernova image from the ground usually blends in with the image of its host galaxy. Hubble can distinguish the light from the two sources and thus measure the supernova directly. For many years cosmologists have discussed whether the expansion of the Universe would stop in some distant future or continue ever more slowly. From the new supernova results it seems clear that the expansion is nowhere near slowing down. In fact, due to some mysterious property of space itself, called dark energy, the expansion is accelerating and will continue forever.

This surprising conclusion came from combined measurements of remote supernovae with most of the world’s top-class telescopes, including Hubble. Furthermore recent supernova results indicate that cosmos did not always accelerate, but began accelerating when the Universe was less than half its current age. The discovery of the accelerating expansion of the Universe led to three astronomers, Saul Perlmutter, Adam Riess and Brian Schmidt, being awarded the 2011 Nobel Prize in Physics . "Hubble gave us the distance measurements of the first four supernovae that made us realise something was wrong with our present understanding of the Universe. Even though the definite proof that the Universe is accelerating came later, we could not reconcile our Hubble observations with a Universe where the expansion is slowing down." Bruno Leibundgut What is a light-year and how is it used?? What is a light-year and how is it used? Answer: A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km.

More p recisely, one light-year is equal to 9,500,000,000,000 kilometers. Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance. In our solar system, we tend to describe distances in terms of the Astronomical Unit (AU). The AU is defined as the average distance between the Earth and the Sun. It is approximately 150 million km (93 million miles).

Mercury can be said to be about 1/3 of an AU from the Sun and Pluto averages about 40 AU from the Sun. The AU, however, is not big enough of a unit when we start talking about distances to objects outside our solar system. For distances to other parts of the Milky Way Galaxy (or even further), astronomers use units of the light-year or the parsec . The light-year we have already defined. The parsec is equal to 3.3 light-years. Using the light-year, we can say that : The Crab supernova remnant is about 4,000 light-years away. The Milky Way Galaxy is about 150,000 light-years across. The Andromeda Galaxy is 2.3 million light-years away. Scale of Universe Measured with 1-Percent Accuracy Scale of Universe Measured with 1-Percent Accuracy By Miriam Kramer, Staff Writer | January 8, 2014 02:16pm ET MORE An artist's concept of the latest, highly accurate measurement of the universe from the Baryon Oscillation Spectroscopic Survey.

The spheres show the current size of the "baryon acoustic oscillations" (BAOs) from the early universe, which have helped to set the distribution of galaxies that we see in the universe today. BAOs can be used as a "standard ruler" (white line) to measure the distances to all the galaxies in the universe. Credit: Zosia Rostomian, Lawrence Berkeley National Laboratory WASHINGTON — An ultraprecise new galaxy map is shedding light on the properties of dark energy, the mysterious force thought to be responsible for the universe's accelerating expansion. A team of researchers working with the Baryon Oscillation Spectroscopic Survey (BOSS) has determined the distances to galaxies more than 6 billion light-years away to within 1 percent accuracy — an unprecedented measurement. "There are not many things in our daily lives that we know to 1-percent accuracy," David Schlegel, a physicist at Lawrence Berkeley National Laboratory and the principal investigator of BOSS, said in a statement.

"I now know the size of the universe better than I know the size of my house." [ The History & Structure of the Universe (Infographic) ] Scientists working with BOSS mapped the locations of 1.2 million galaxies and found that their new measurements support the idea of the "cosmological constant" — an idea first proposed by Albert Einstein . This idea suggests that dark energy has remained constant throughout the history of the universe. "We don’t yet understand what dark energy is, but we can measure its properties," Daniel Eisenstein, a Harvard University astronomer working with the survey, said in a statement. "Then, we compare those values to what we expect them to be, given our current understanding of the universe. The better our measurements, the more we can learn." The new results, presented by Schlegel and his colleagues here today (Jan. 8) at the 223rd meeting of the American Astronomical Society, also provide one of the best-ever determinations of the curvature of space, researchers said.

In short, the universe appears to be quite "flat," meaning that its shape can be described well by Euclidean geometry, in which straight lines are parallel and the angles in a triangle add up to 180 degrees. "One of the reasons we care is that a flat universe has implications for whether the universe is infinite," Schlegel said. "That means — while we can't say with certainty that it will never come to an end — it's likely the universe extends forever in space and will go on forever in time. Our results are consistent with an infinite universe." BOSS astronomers use a spectrograph on the Sloan Foundation's 2.5-meter telescope at Apache Point Observatory in New Mexico to make their observations. "On a clear night when everything goes perfectly, we can add more than 8,000 galaxies and quasars to the map," Kaike Pan, who leads the team of observers working with the spectrograph, said in a statement. The team has presented other results from the galaxy survey . However, the new map extends the survey's reach, more than doubling the size of the examined area and thereby allowing for more precise measurements. The newly presented data also include earlier results that surveyed nearby galaxies, BOSS officials said.

"Making these measurements at two different distances allows us to see how the expansion of the universe has changed over time, which will help us understand why it is accelerating," explained University of Portsmouth astronomer Rita Tojeiro, who co-chairs the BOSS galaxy clustering working group along with Jeremy Tinker of New York University. The new results should help lay the foundation for even more-precise measurements in the coming years, Schlegel said. "Future surveys will be doing more of this, both filling in this map of the universe, [and] the enormous volume of the universe we have yet to map out and doing this with even higher precision," Schlegel said here today at the AAS meeting.

Question: What units are used for measuring distances in the universe?
Answer: Light years