Invisible X-rays: This fuzzy orb of light is a giant elliptical galaxy filled with an incredible 200 billion stars. Unlike spiral galaxies, which have a well-defined structure and boast picturesque spiral arms, elliptical galaxies appear fairly smooth and featureless. This is likely why this galaxy, named Messier 49, was discovered by French astronomer Charles Messier in 1771. At a distance of 56 million light-years, and measuring 157 000 light-years across, M49 was the first member of the Virgo Cluster of galaxies to be discovered, and it is more luminous than any other galaxy at its distance or nearer. Elliptical galaxies tend to contain a larger portion of older stars than spiral galaxies and also lack young blue stars. Messier 49 itself is very yellow, which indicates that the stars within it are mostly older and redder than the Sun. In fact, the last major episode of star formation was about six billion years ago — before the Sun was even born! Messier 49 is also rich in globular clusters; it hosts about 6000, a number that dwarfs the 150 found in and around the Milky Way. On average, these clusters are 10 billion years old. Messier 49 is also known to host a supermassive black hole at its centre with the mass of more than 500 million Suns, identifiable by the X-rays pouring out from the heart of the galaxy (as this Hubble image comprises infrared observations, these X-rays are not visible here).

Name: Messier 49
Type: Local Universe : Galaxy : Type : Elliptical
Category: Galaxies
Constellation: Virgo
Distance: 56 million light years
Release date: 18 March 2019, 06:00
Credit: ESA/Hubble & NASA, J. Blakenslee, P Cote et al.

Wild cosmic ducks: This star-studded image shows us a portion of Messier 11, an open star cluster in the southern constellation of Scutum (The Shield). Messier 11 is also known as the Wild Duck Cluster, as its brightest stars form a “V” shape that somewhat resembles a flock of ducks in flight. Messier 11 is one of the richest and most compact open clusters currently known. By investigating the brightest, hottest main sequence stars in the cluster astronomers estimate that it formed roughly 220 million years ago. Open clusters tend to contain fewer and younger stars than their more compact globular cousins, and Messier 11 is no exception: at its centre lie many blue stars, the hottest and youngest of the cluster’s few thousand stellar residents. The lifespans of open clusters are also relatively short compared to those of globular ones; stars in open clusters are spread further apart and are thus not as strongly bound to each other by gravity, causing them to be more easily and quickly drawn away by stronger gravitational forces. As a result Messier 11 is likely to disperse in a few million years as its members are ejected one by one, pulled away by other celestial objects in the vicinity.

Name: Messier 11
Type: Milky Way : Star : Grouping : Cluster : Open
Category: Star Clusters
Constellation: Scutum
Distance: 6000 light years
Release date: 25 March 2019, 06:00
Credit: ESA/Hubble & NASA, P. Dobbie et al.

The largest of its kind: Star clusters are commonly featured in cosmic photoshoots, and are also well-loved by the keen eye of the NASA/ESA Hubble Space Telescope. These large gatherings of celestial gems are striking sights — and the subject of this Picture of the Week, Messier 2, is certainly no exception. Messier 2 is located in the constellation of Aquarius (The Water-Bearer), about 55 000 light-years away. It is a globular cluster, a spherical group of stars all tightly bound together by gravity. With a diameter of roughly 175 light-years, a population of 150 000 stars, and an age of 13 billion years, Messier 2 is one of the largest clusters of its kind and one of the oldest associated with the Milky Way. Most of the cluster’s mass is concentrated at its centre, with shimmering streams of stars extending outwards into space. It is bright enough that it can even be seen with the naked eye when observing conditions are extremely good.

Name: Messier 2
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Star Clusters
Constellation: Aquarius
Distance: 55000 light years
Release date: 1 April 2019, 06:00
Credit: ESA/Hubble & NASA, G. Piotto et al.

Blue rejuvenation: Globular clusters are inherently beautiful objects, but the subject of this NASA/ESA Hubble Space Telescope image, Messier 3, is commonly acknowledged to be one of the most beautiful of them all. Containing an incredible half a million stars, this eight-billion-year-old cosmic bauble is one of the largest and brightest globular clusters ever discovered. However, what makes Messier 3 extra special is its unusually large population of variable stars — stars that fluctuate in brightness over time. New variable stars continue to be discovered in this sparkling stellar nest to this day, but so far we know of 274, the highest number found in any globular cluster by far. At least 170 of these are of a special variety called RR Lyrae variables, which pulse with a period directly related to their intrinsic brightness. If astronomers know how bright a star truly is based on its mass and classification, and they know how bright it appears to be from our viewpoint here on Earth, they can thus work out its distance from us. For this reason, RR Lyrae stars are known as standard candles — objects of known luminosity whose distance and position can be used to help us understand more about vast celestial distances and the scale of the cosmos. Messier 3 also contains a relatively high number of so-called blue stragglers, which are shown quite clearly in this Hubble image. These are blue main sequence stars that appear to be young because they are bluer and more luminous than other stars in the cluster. As all stars in globular clusters are believed to have formed together and thus be roughly the same age. Only a difference in mass can give these stars a different colour: a red, old star can appear bluer when it acquires more mass, for instance stripping it from a nearby star. The extra mass changes it into a bluer star, which makes us think it is younger than it really is.

Name: Messier 3
Type: Milky Way : Star : Grouping : Cluster : Globular
Category: Star Clusters
Constellation: Canes Venatici
Distance: 34000 light years
Release date: 8 April 2019, 06:00
Credit: ESA/Hubble & NASA, G. Piotto et al.