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Picture Details:

   Optics            32-inch Schulman Telescope (RC Optical Systems), Acquired remotely
   Camera            SBIG STX 16803 CCD Camera
   Filters           AstroDon Gen II
   Dates             May 2013
   Location          Mount Lemmon SkyCenter
   Exposure          RGB = 90:90:90 minutes
   Acquisition       ACP Observatory Control Software (DC-3 Dreams), Maxim DL/CCD (Cyanogen), FlatMan XL (Alnitak)
   Processing        CCDStack (CCDWare), Photoshop CC (Adobe), PixInsight
   Credit Line and Copyright        Adam Block/Mount Lemmon SkyCenter/University of Arizona 

This new NASA/ESA Hubble Space Telescope image shows the globular cluster IC 4499.

Globular clusters are big balls of old stars that orbit around their host galaxy. It has long been believed that all the stars within a globular cluster form at the about same time, a property which can be used to determine the cluster's age. For more massive globulars however, detailed observations have shown that this is not entirely true — there is evidence that they instead consist of multiple populations of stars born at different times. One of the driving forces behind this behaviour is thought to be gravity: more massive globulars manage to grab more gas and dust, which can then be transformed into new stars.

IC 4499 is a somewhat special case. Its mass lies somewhere between low-mass globulars, which show a single generation build-up, and the more complex and massive globulars which can contain more than one generation of stars. By studying objects like IC 4499 astronomers can therefore explore how mass affects a cluster's contents. Astronomers found no sign of multiple generations of stars in IC 4499 — supporting the idea that less massive clusters in general only consist of a single stellar generation.

Hubble observations of IC 4499 have also helped to pinpoint the cluster's age: observations of this cluster from the 1990s suggested a puzzlingly young age when compared to other globular clusters within the Milky Way. However, since those first estimates new Hubble data been obtained, and it has been found to be much more likely that IC 4499 is actually roughly the same age as other Milky Way clusters at approximately 12 billion years old.

Credit:

ESA/Hubble & NASA

About the Object

Name:	IC 4499
Type:	• Milky Way : Star : Grouping : Cluster : Globular
       • X - Stars Images/Videos
Distance:	55000 light years

Colours & filters Band Wavelength Telescope

Optical V 	606 nm 	Hubble Space Telescope WFC3
Infrared I 	814 nm 	Hubble Space Telescope WFC3

The MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile captured this richly colourful view of the bright star cluster NGC 3532. Some of the stars still shine with a hot bluish colour, but many of the more massive ones have become red giants and glow with a rich orange hue.

Credit:

ESO/G. Beccari

About the Object

Name:	NGC 3532
Type:	• Milky Way : Star : Grouping : Cluster : Open
        • X - Star Clusters
Distance:	1300 light years
Constellation:	Carina

Colours & filters Band Telescope

Optical B	 MPG/ESO 2.2-metre telescope WFI
Optical V	 MPG/ESO 2.2-metre telescope WFI
Optical H-alpha MPG/ESO 2.2-metre telescope WFI
Infrared I	 MPG/ESO 2.2-metre telescope WFI

Colours & filters
Band Telescope
Optical B MPG/ESO 2.2-metre telescope WFI
Optical V MPG/ESO 2.2-metre telescope WFI

This brilliant image of Messier 30 (M 30) was taken by Hubble's Advanced Camera for Surveys (ACS). Messier 30 formed 13 billion years ago and was discovered in 1764 by Charles Messier. Located about 28000 light-years away from Earth, this globular cluster — a dense swarm of several hundred thousand stars — is about 90 light-years across.

Messier 15 is a globular cluster — a spherical conglomeration of old stars that formed together from the same cloud of gas, found in the outer reaches of the Milky Way in a region known as the halo and orbiting the Galactic Centre. This globular lies about 35 000 light-years from the Earth, in the constellation of Pegasus (The Flying Horse).

Messier 15 is one of the densest globulars known, with the vast majority of the cluster’s mass concentrated in the core. Astronomers think that particularly dense globulars, like this one, underwent a process called core collapse, in which gravitational interactions between stars led to many members of the cluster migrating towards the centre.

Messier 15 is also the first globular cluster known to harbour a planetary nebula, and it is still one of only four globulars known to do so. The planetary nebula, called Pease 1, can be seen in this image as a small blue blob to the lower left of the globular’s core.

Stars in globular clusters form in the same stellar nursery and grow old together. The most massive stars age quickly, exhausting their fuel supply in less than a million years, and end their lives in spectacular supernovae explosions. This process should have left the ancient cluster Messier 5 with only old, low-mass stars, which, as they have aged and cooled, have become red giants, while the oldest stars have evolved even further into blue horizontal branch stars.

Yet astronomers have spotted many young, blue stars in this cluster, hiding amongst the much more luminous ancient stars. Astronomers think that these laggard youngsters, called blue stragglers, were created either by stellar collisions or by the transfer of mass between binary stars. Such events are easy to imagine in densely populated globular clusters, in which up to a few million stars are tightly packed together.

Picture Details:

   Optics            24-inch RC Optical Systems Telescope
   Camera            SBIG STL11000 CCD Camera
   Filters           Custom Scientific
   Dates             May 30th 2009
   Location          Mount Lemmon SkyCenter
   Exposure          LRGB = 25:25:25:25 minutes
   Acquisition       ACP Observatory Control Software (DC-3 Dreams),TheSky (Software Bisque), Maxim DL/CCD (Cyanogen)
   Processing        CCDStack (CCDWare), Mira (MiraMetrics), Photoshop CS3 (Adobe)
   Guest Astronomers:                   May 2009 CCD Image Processing Participants

These enormous sparkling spheres are by no means rare, and over 150 exist in the Milky Way alone, including Messier 53. It lies on the outer edges of the galaxy, where many other globular clusters are found, almost equally distant from both the centre of our galaxy and the Sun. Although they are relatively common, the famous astronomer William Herschel, not at all known for his poetic nature, once described a globular cluster as “one of the most beautiful objects I remember to have seen in the heavens”, and it is clear to see why.

Globular clusters are much older and larger than open clusters, meaning they are generally expected to contain more old red stars and fewer massive blue stars. But Messier 53 has surprised astronomers with its unusual number of a type of star called blue stragglers.

These youngsters are rebelling against the theory of stellar evolution. All the stars in a globular cluster are expected to form around the same time, so they are expected follow a specific trend set by the age of the cluster and based on their mass. But blue stragglers don’t follow that rule; they appear to be brighter and more youthful than they have any right to be. Although their precise nature remains mysterious these unusual objects are probably formed by close encounters, possibly collisions, between stars in the crowded centres of globular clusters.