Data From the Virtual Universe
Brian Abbott, Carter Emmart & Ryan Wyatt
Virtual Universe
Hayden Planetarium
Fall 2001

Stellar Data

De nition: A star is a ball of gas that shines from nuclear reactions in its
interior. The story's much more complicated, but you know what one is! See
any introductory astronomy text for an through explanation.

The stellar database is one of the more challenging data sets, primarily
because distances can be so diÆcult to calculate. The distance to a star canbe calculated using two methods. One method uses the trigonometric parallax, an angle we measure which is related to the star's apparent movementin the sky as we revolve around the Sun. Even for the closest stars, this is anextremely small angle and, therefore, is only measurable for stars near theEarth (and Sun). If we can measure the angle, the distance is calculated via
the equation
1d= :(1)Here  is the trig parallax angle that we measure, and the distance, d, is inparsecs. If we can't measure the parallax, we use the brightness of the stars
to get the distance usingd = 10(m M +5)=5 ;(2)where m is the apparent magnitude, the brightness of the star as seen fromEarth, and M is the absolute magnitude, the intrinsic brightness of the starinferred from the spectral type.
The Hipparcos catalogue has 118,218 stars. From these, we can calculate
good distances to about 25,000 stars|enough to give us the familiar view ofthe night sky as seen from Earth.

Examples: innumerable.
The Hipparcos and Tycho Catalogs

ESA, 1997, The Hipparcos Catalogue, ESA SP-1200
ESA, 1997, The Tycho Catalogue, ESA SP-1200

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/1/1239/

1Figure 1: A diagram of the parallax method of distance determination. Theparallax angle is that formed by the apparent motion of the star in the sky.

2Extra-Solar Planets

De nition: Extra-solar planets, or exoplanets, are plamets that orbit stars

other than our Sun. The discovery of the rst con rmed extra-solar planet
was in 1995 (51 Peg). Since then, there have been 71 more discovered (asof October 6, 2001). These orbiting objects are Jupiter-like planets|large,
gaseous bodies that orbit their host star.

Most exoplanets are not seen through telescopes, they are inferred fromthe motion of the parent star. The action of a planet revolving around its
parent star is actually both the star and the planet orbiting around a common
center of mass. For the Earth-Sun orbit, the center of mass is a point on theline connecting the two bodies about 447 km (279 miles) from the center ofthe Sun. For Jupiter, this distance is 743,100 km from the Sun's center, justoutside the Sun's \surface" (the Sun's radius is 696,000 km)! The motion ofthe star about the center of mass is detected in the stellar spectrum, wherethe spectral lines are Doppler shifted.

Examples: 51 Pegasi ( rst con rmed star with its own planets)
Extra-Solar Planets Catalog

Schneider J. 1998 (but discovered by many)
Unpublishedhttp://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/external/combined/E5002/
http://cfa-www.harvard.edu/planets/catalog.html
http://cfa-www.harvard.edu/planets/

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Open Clusters

De nition: An open cluster is a small group (tens to hundreds) of stars

that are gravitationally bound with one another. They are young stars thathave just been born. These clusters are found generally in the plane of theGalaxy, since star formation is occurring mainly in the Galactic arms.
We observe many open clusters in our Galaxy, each in a di erent stageof evolution. The cluster of stars in the Orion Nebula are in the birthing
stage where we literally can see stars being born. The stars of the Pleiades(M45) are more evolved|only traces of the nebula from which the stars wereformed can be seen. While some open clusters are much older and have swept
away all the gas from which they formed.

Examples: The Pleiades (M45), the Hyades in Taurus, the Beehive (M44)
in Cancer, and many, many more.

Open Cluster Data 5th Edition
Lynga G.
Lund Observatory (1987)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7092A/

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Globular Clusters

De nition: A globular cluster is a gravitationally bound group of one hun-dred thousand to one million stars. They are compact, spherically-symmetric\balls" of stars, with some member stars being almost as old as the universe
itself! Globular clusters orbit the Milky Way galaxy (just as a comet revolves around the Sun) with orbital periods near 100 million years. They arewithin the Galactic Halo, a sphere centered around the Galaxy with a radiusof about 100 kpc.
The distances to the clusters were inferred from the mean V magnitude(visual brightness), mV , of the RR Lyrae stars in the cluster. RR Lyrae stars
are giant, pulsating variable stars (their brightness varies periodically). We
observe some RR Lyrae stars relatively close to the Sun (and the Earth)|
close enough to get some of the star's fundamental properties, like its absolutemagnitude, M . We can then calculate the distance to the star via the equation m M = 5 log d 5. Since the light from the RR Lyrae star has adistinct variability, if we see one across the Galaxy, we can infer its absolutemagnitude and calculate its distance. Since these stars are commonly foundin globular clusters, its quite easy to derive their distance from observations.
The globular cluster data form one of the most complete data sets wehave. The data we used contains 145 entries, just short of the total number
we expect in the Galaxy.

Examples: M13 in Hercules, 47 Tuc in Tucana, M4 in Scorpius
Globular Clusters in the Milky Way

William E. Harris, McMaster University, Hamilton, ON, Canada
Astronomical Journal, v112, pp1487 (1997)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7202/
http://physun.physics.mcmaster.ca/Globular.html

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Pulsars

De nition: Pulsars are objects that give o periodic pulses of radio waves.

The periods of these pulses range from 0.001 to 4.0 seconds with an averageof 0.65 seconds. Physically, they are the dying remnants of stars whose initial
mass was 1.4{3.0 solar masses. These stellar corpses are called neutron stars.
These stars are so dense that protons and electrons are literally squeezed
into neutrons, forming a neutron gas. This gas has such a high density, 1017
kg/m3 (that's a 1 with 17 zeros; compare to the average density of the Earthat 5,520 kg/m3 ), that one teaspoon full would weigh about a billion tons onearth. On top of this, they are only a few tens of kilometers in diameter(about the size of the NYC area).
Astrophysicists use the lighthouse model to describe the pulse of radiation
we see. As the star rotates, the source of the \pulse" comes in and out of ourview like a light from a lighthouse. But why do they pulse? Neutron stars
have a strong magnetic eld which is typically not aligned with the axis ofrotation, as it is on Earth where the North Magnetic Pole is in the tundra ofCanada. Charged particles, or electrons, escaping from the star travel up themagnetic eld lines, giving o radiation in the process. We see this radiationin radio wavelengths (the rst pulsar was discovered in 1967 at a frequency of
81.5 MHz, just below the FM range which begins at 87.5 MHz). The sourceof the radio emission is constant and is directed in a cone from the magnetic
poles, but we only see this emission when it rotates into view. Therefore, theperiod of the pulsar allows us to determine its rotation rate.
Deducing the distance to a pulsar is not trivial. If pulses of two di erent
frequencies f1 and f2 are emitted at some time t0 , the times when theyarrive at earth will be di erent. This is because of the material (electrons)in the line of sight slowing the light signal down, just as glass slows lightdown (described by its index of refraction). As we look at lower frequency
pulses of radio emission, the signal takes longer and longer to reach theEarth. Therefore, the times these two pulses f1 and f2 reach Earth will be
t1 t0 = d=v1 and t2 t0 = d=v2 . We don't know t0 but we can measure
t2 t1 and so the two equations above become
1 1
t2 t1 =d:(3)v2 v1The velocities, v1 and v2 , depend on the density of electrons between us
and the pulsar. The quality of the calculated distance is then subject to6this density measurement. So, if we know the electron density and measure
the times between two pulses of di erent frequencies, we can calculate thepulsar's distance.

Examples: The Crab pulsar (PSR 0531+21), the Vela pulsar (PSR 083345), PSR 1937+21, the fastest pulsar at 1.56 milliseconds (or 640 times persecond!).

Taylor: Catalog of 558 Pulsars

Updated May 1995 to 706 pulsars
Taylor J. H., Manchester R. N., Lyne A. G.
Astrophysical Journal Supplement Series, v88, pp529 (1993)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7156A/

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Dark Nebulae

De nition: Dark nebulae are clouds of dust in the interstellar medium which

obscure the light behind them. The Coal Sack in the southern hemisphere,
the Great Rift in Cygnus, and the Horsehead Nebulae in Orion are all good
examples. Dust is produced in the atmospheres of cooler giant stars or dense
clouds where the temperatures are low enough to allow for the condensationof silicates. These dust grains are not the ordinary household dust we're used
to seeing, these particles are so small you would need a microscope to seethem.
These data are very spotty. Based on the Lynds catalogue of 1962, Palomar plates (observed from the Palomar Observatory in California) were used
to map dark nebulae in the sky. However, one cannot see the southern skyfrom California so there is little or no coverage in the southern sky. WhileLynds mapped the dark nebulae in two dimensions, it was Hilton and Lahullain the 1990s who searched the literature to nd distances to each of them.
Many are found using the light from stars in the line of sight. The lightfrom closer stars are a ected less than the light from stars farther away (andcloser to the dust cloud). If the distances to the stars are known, then thedark nebula can be placed among them.

Examples: Horsehead Nebula in Orion, the Coal Sack in Crux (SouthernCross).

The Lynds Catalogue of Dark Nebulae

Lynds B. T.
Astrophysical Journal Supplement Series, v7, pp1 (updated version) (1962)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7007A/

Distance measurements of Lynds galactic dark nebulae
Hilton J., Lahulla J. F.
Astronomy & Astrophysics Supplement, v113, pp325 (1995)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/journal tables/A+AS/113/325/

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H II Regions

De nition: An H ii region is an area of glowing, ionized hydrogen gas sur-rounding young, hot stars. Ionized gas is so hot that the hydrogen atoms
have been stripped of their electron. They have a temperature of about
10,000 Kelvin and a density of about 5000 particles/m3. The star's light (orradiation) illuminates the hydrogen gas, causing it to glow, like the Orion
Nebula. Further away from the star, the light is \used up" in exciting thenearby gas and, the hydrogen is in it's non-glowing, non-ionized state called
neutral hydrogen, or H i.
H ii regions can be seen in many parts of the electromagnetic spectrum.
The Orion Nebula provides us with a good example. We observe H ii gas in
the visible, star formation in the infrared, and hot stellar winds in the X-ray
part of the spectrum. However, an H ii region is just a small part of thecloud.
H i clouds which surround H ii regions are observed primarily in the radio
part of the electromagnetic spectrum. Because radio waves penetrate theinterstellar dust and gas, it is really the neutral hydrogen, H i, that aids us in
seeing the clouds in which the H ii region lies. Information from these radio
observations give us the general properties of the hydrogen cloud, its density,and its motion in the galaxy. In this way, we can observe the rotation of ourown galaxy and, using both H i and H ii data, we can map its spiral structure.

Examples: Orion Nebula (M42), Lagoon Nebula (M8) in Sagittarius
CO Radial Velocities Toward Galactic H II Regions (Blitz+ 1982)
Catalog of CO Radial Velocities toward Galactic H II Regions
Blitz L., Fich M., Stark A.A.
Astrophysical Journal Supplement Series, v49, pp183 (1982)

CO Observations of Southern Hemisphere H II Regions

Gillespie A.R., et al
Astronomy & Astrophysics, v60, pp221 (1977)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7050/

Georgelin & Georgelin on the spiral structure of our Galaxy

Georgelin, Y. M. & Georgelin, Y. P.
Astronomy & Astrophysics, v49, pp57 (1976)
Not published in electronic form.
9Planetary Nebulae

De nition: A planetary nebula is a shell of expanding gas illuminated bya central star. While H ii regions are the sign of stellar birth, planetarynebulae are a signature of stellar death. As a star similar to our Sun evolves,it eventually will run out of nuclear fuel (hydrogen) in the core and will begin
burning helium. The core will eventually become all helium which will thenbe converted into carbon as it burns. Once the core is pure carbon, the star
will become unstable and blow out its outer layers. A spherical shell of gas
forms around what is now the exposed core of the central star. The core willquickly become a white dwarf|a small, dense star that no longer shines by
nuclear burning but stored residual energy. This is the fate of stars that havea mass less than 1.4 solar masses (like our Sun). The star will continue toradiate this energy sparingly for billions of years. We call the spherical shellof gas a planetary nebula.
Planetary nebulae are found mainly in the disk and the center of theGalaxy. However, their orbits are elongated around the Galactic center. Inaddition, some are found above the plane, and one in a globular cluster even.

Examples: The Ring Nebula (M57) in Lyra, Helix Nebula (NGC 7293) inAquarius, the Dumbbell Nebula (M27) in Vulpecula

Strasbourg-ESO Catalogue of Galactic Planetary Nebulae

Acker A., Ochsenbein F., Stenholm B., Tylenda R., Marcout J., Schohn C.
European Southern Observatory|ISBN 3-923524-41-2 (1992)
http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/5/5084/

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Galaxies

De nition: A galaxy is a huge assemblage of stars (our Milky Way has

about 1012 stars), gas, and dust that is held together by gravity. All of what
we see in the sky with our naked eye, with the exception of the Andromeda
galaxy (M31), is in our Galaxy.

This data set has been provided to us directly via the author, Brent Tullyof the University of Hawaii. These data consist of over 28,000 galaxies thatsurround our Milky Way. In it, we see the structure of the local universe|laments of galaxies that intersect to form clusters of galaxies. These data
form two lobes on either side of the Galaxy. The entire data set is about
300 million light-years across a side (i.e., the data forms a cube-like structurewith each side being about 300 million light-years). In the middle is theMilky Way, in what is called the Zone of Obscuration. This zone forms theempty space in between the two lobes. What this zone represents is the planeof our own Galaxy. Because we cannot see through our Galaxy, there is nodata on either side of the Milky Way, only above and below the plane of theGalaxy. This relates to the forest through the trees argument, there are just
too many trees in the Milky Way to see through it and out into the universe.
Each galaxy has been given a color based on which cluster or lament
they belong to. The color code is simply a way to see more structure as wemove through it in three dimensions. As we pull away from the Milky Way,
we can see the Local Group cluster members and we can see that the Local
Group galaxies are a member of the Virgo Cluster. As we open our eyes tomore galaxies, we see the web-like structure of the Universe. The distances
to these galaxies are measured by redshift of their spectral lines, which will
be discussed next week.

Examples: Any galaxy outside our own Milky Way
Nearby Galaxies Catalogue

Tully, R. B., University of Hawaii
Cambridge University Press (1988)

http://adc.gsfc.nasa.gov/adc-cgi/cat.pl?/catalogs/7/7145/

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