HEIC: M15: Old Stars with a Youthful Glow

[bystander]

Old Stars with a Youthful Glow
ESA/HEIC Hubble Picture of the Week | 2011 Feb 14

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The dazzling stars in Messier 15 look fresh and new in this image from the NASA/Hubble Space Telescope, but they are actually all roughly 13 billion years old, making them some of the most ancient objects in the Universe. Unlike another recent Hubble Picture of the Week, which featured the unusually sparse cluster Palomar 1, Messier 15 is rich and bright despite its age.

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.

This picture was put together from images taken with the Wide Field Channel of Hubble's Advanced Camera for Surveys. Images through yellow/orange (F606W, coloured blue) and near-infrared (F814W, coloured red) filters were combined. The total exposure times were 535 s and 615 s respectively and the field of view is 3.4 arcminutes across.

Credit: NASA/ESA/Hubble

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FB: Hubble European Picture of the Week
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[bystander]

A buzzing beehive and a dying star
Discover Blogs | Bad Astronomy | 2011 Feb 15

When I was younger, it was pretty common on clear nights to see me at the end of my driveway with my telescope. And one of my favorite targets to observe was (and still are) globular clusters: hundreds of thousands, and sometimes millions of stars all bound together in a tight ball due to their gravity. And one of the best of those is the fabulous M15… and when it’s seen by Hubble, well, it’s simply spectacular: (see previous post)

Holy wow! Click to englobulenate – I had to shrink the image a lot to get it to fit here, so as gorgeous as this is it’s a shadow of the higher-res version… or the ginormous full-res one!

M15 is relatively nearby as globulars go, about 35,000 light years. Over 150 of these objects orbit our galaxy, and so some are quite far away. Not only is it close, but M15 is also fairly densely-populated, its stars orbiting each other like bees around a beehive, making it a pretty easy target for amateur astronomers. It was one of the first things I’d go after once it got dark in the autumn, and it would appear as a fuzzy ball in my 25 cm ‘scope. Of course, when you aim the 2.4 meter mirror of Hubble at it, well. You can see for yourself.

This false-color image is a combination of two pictures; one taken in visible light (colored blue; in reality the filter used let through yellow and red light), and the other in near-infrared (colored red). That selects out redder stars; the brightest ones are red giants, stars nearing the ends of their lives, and the fainter ones are lower-mass stars that are still busily fusing hydrogen into helium in their cores.

If you look to the left and a bit below the cluster’s center, though, a blue glow sticks out among all the red. If you do grab the seriously super high-res version of the image, you get a much better look at it. I’ve zoomed in on it here. It’s clearly not a star; the blue halo is much larger than any star image, and you can see the rim on the left hand side is bright. What gives?

This is a planetary nebula, the shell of gas expelled by a star as it really starts to give up the ghost. Many of those red giant stars in the cluster are blowing off a slow, dense, wind of material, far too faint to see. But after a few thousand years, so much material is lost that by a red giant that the hot core is exposed, and a faster wind starts to blow. This catches up to and slams into the red giant wind, compressing it. The ultraviolet light from the core of the star lights up the gas, causing it to glow.

We see hundreds of these nebulae in the sky, but they don’t last long, just a few thousand years, and not too many are seen in globular clusters. This one, named Pease 1, was actually the first such detected in a globular. It’s a challenging object using just a telescope and your eye, but shows up readily in images. And this Hubble image is the finest I’ve seen! You can actually see some structure in the gas, which is a remarkable achievement given its distance of 35,000,000,000,000,000 kilometers! That’s 21 quadrillion miles, if you like your units that way.

Eventually that gas will diffuse and merge with the ethereally thin stuff between the stars in that cluster, and the star will fade. Such is the fate of all the stars in the cluster, actually, so take a good look now and appreciate what you can see. In a couple of hundred billion years it’ll be gone.