Star Cluster

WISE Reveals a Hidden Star Cluster


There are few things in astronomy more awe inspiring and spellbinding than the birth of a star. Even though we now understand how they are formed, the sheer magnitude of it is still enough to stir the imagination of even the most schooled and cynical academics. Still, there is some degree of guesswork and chance when it comes to where stars will be born and what kind of stars they will become. For example, while some stars are single field stars (like our Sun), others form in groups of two (binary) or more, sometimes much more. This is what is known as a Star Cluster, by definition, a group of stars that share a common origin and are gravitationally bound for some length of time.

Thereare two basic categories of star clusters: Globular and Open (aka. Galactic) star clusters. Globular clusters are roughly spherical groupings of stars that range from 10,000 to several million stars packed into regions ranging from 10 to 30 light years across. They commonly consist of very old Population II stars – which are just a few hundred million years younger than the universe itself – and are mostly yellow and red. Open clusters, on the other hand, are very different. Unlike the spherically distributed globulars, open clusters are confined to the galactic plane and are almost always found within the spiral arms of galaxies. They are generally made up of young stars, up to a few tens of millions of years old, with a few rare exceptions that are as old as a few billion years. Open clusters also contain only a few hundred members within a region of up to about 30 light-years. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, will become disrupted by the gravity of giant molecular clouds and other clusters.

Star clusters are particularly useful to astronomers as they provide a way to study and model stellar evolution and ages. By estimating the age of globular clusters, scientists were able to get a more accurate picture of how old the universe is, putting it at roughly 13 billion years of age. In addition, the location of star clusters and galaxies is believed to be a good indication of the physics of the early universe. This is based on aspects of the Big Bang theory where it is believed that immediately after the creation event, following a period of relatively homogenous distribution; cosmic matter slowly gravitated to areas of higher concentration. In this way, star clusters and the position of galaxies provide an indication of where matter was more densely distributed when the universe was still young.

Some popular examples of star clusters, many of which are visible to the naked eye, include Pleiades, Hyades, the Beehive Cluster and the star nursery within the Orion Nebula.

We have written many articles about star cluster for Universe Today. Here’s an article about a massive star cluster discovered, and here are some amazing star cluster wallpapers.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?


Dating a Cluster – A New Trick


Finding the ages of things in astronomy is hard. While it is undoubted that the properties of objects change as they age, the difficulty lies in that the initial parameters are often so varied that, for most cases, finding reliable ages is challenging. There’s some tricks to do it though. One of the best ones, taught conceptually in introductory astronomy courses, is to use the “main sequence turn-off” of a cluster. Of course, applying any of these methods is easier said than done, but a new method may help alleviate some of the challenges and allow for smaller errors.

The largest difficulty in the main sequence turn-off method lies in the inherent scatter caused by numerous sources that must be accounted for. Stars that lie along the same line of sight as the cluster being observed can add extraneous data points. Any interstellar reddening caused by gas may make stars appear more red than they should be. Close binary stars that cannot be spatially resolved appear brighter than they should be as an individual star. The amount of heavy elements in the star will also effect the fitting of the model. All of these factors and more contribute to an uncertainty in any calculation that requires an accurate Hertzsprung-Russell Diagram. Tricks to correct for some of these factors exist. Others cannot (yet) be accounted for.

Thanks to all these problems, fitting the data can often be challenging. Finding the point where the cluster “peels away” from the main sequence is difficult, so one of the tricks is to look for other points that should have significant numbers of stars to provide extra reference points for fitting. Examples of this include the horizontal branch and the red clump.

The new technique, developed by a large team of international astronomers, uses “a well defined knee located along the lower main sequence” which they refer to as the Main Sequence Knee (MSK). This “knee” appears in H-R diagrams of the clusters taken in the near-infrared and is largely independent of the age of the cluster. As such, it provides a stable reference point to improve corrections for the general main sequence turn-off method. Additionally, since this system uses infrared wavelengths, it is less prone to contamination between gas and dust.

To test this new method, the group selected a globular cluster (NGC 3201) as a test case. When their method was applied, they found that their derived age for the cluster was consistent with ages derived by other methods.

However, the new method is not without difficulties of its own. Since the knee is at the faint end of the main sequence, this requires that exposure times for target clusters be sufficiently long to bring out such faint stars. Fortunately, with new telescopes like the the James Webb Space Telescope, these faint stars should be in reach.

Telescopes Open Up the Jewel Box

Nothing in my jewelry box compares to the Kappa Crucis Cluster, also known as NGC 4755 or simply the “Jewel Box.” This object is just bright enough to be seen with the unaided eye, but a combination of images taken by three exceptional telescopes, the Very Large Telescope, the 2.2-meter telescope at the La Silla observatory and the Hubble Space Telescope, has allowed the stunning Jewel Box star cluster to be seen in a whole new light. Above is the image from ESO’ Very Large Telescope, which zooms in for a close look at the cluster itself. This new image is one of the best ever taken of this cluster from the ground, taken with an exposure time of just 5 seconds.

A Hubble gem: the Jewel Box.  Credit: NASA/ESO
A Hubble gem: the Jewel Box. Credit: NASA/ESO

The Hubble Space Telescope can capture light of shorter wavelengths than ground-based telescopes can, and this new HST image of the core of the cluster represents the first comprehensive far ultraviolet to near-infrared image of an open galactic cluster. It was created from images taken through seven filters, allowing viewers to see details never seen before. It was taken near the end of the long life of the Wide Field Planetary Camera 2, Hubble’s workhorse camera up until the recent Servicing Mission, when it was removed and brought back to Earth, and replaced with an new and improved version. Several very bright, pale blue supergiant stars, a solitary ruby-red supergiant and a variety of other brilliantly colored stars are visible in the Hubble image, as well as many much fainter ones. The intriguing colors of many of the stars result from their differing intensities at different ultraviolet wavelengths.

Wide Field Image  of the Jewel Box.  Credit:  ESO
Wide Field Image of the Jewel Box. Credit: ESO

A new image taken with the Wide Field Imager (WFI) on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile shows the cluster and its rich surroundings in all their multicolored glory. The large field of view of the WFI shows a vast number of stars. Many are located behind the dusty clouds of the Milky Way and therefore appear red.

Composite image of the Jewel Box. Credit: ESO
Composite image of the Jewel Box. Credit: ESO

Star clusters are among the most fascinating objects in the sky. Open clusters such as NGC 4755 typically contain anything from a few to thousands of stars that are loosely bound together by gravity. Because the stars all formed together from the same cloud of gas and dust their ages and chemical makeup are similar, which makes them ideal laboratories for studying how stars evolve.

Source: ESO