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astronomy

With Age Comes Beauty

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Some Puzzling Stellar Plastic Surgery

his colourful view of the globular star cluster NGC 6362 was captured by the Wide Field Imager attached to the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. This new picture, along with a new image of the central region from the NASA/ESA Hubble Space Telescope, provide the best view of this little-known cluster ever obtained. Globular clusters are mainly composed of tens of thousands of very ancient stars, but they also contain some stars that look suspiciously young.

Want to see some of the oldest stars in the cosmos?  Follow European Southern Observatory’s gaze out to the globular star cluster NGC 6362.  It belongs to the Milky Way, and contains tens of thousands of very ancient stars. This cluster has many stars that have aged to become red giants. But, there are some stars here that look–well — almost young. Blue. Hot.

Those blueish hotties are called “blue stragglers” and they’re passing as younger stars.

How could this be?

Astronomers know that all of the stars in a globular cluster formed from the same material at roughly the same time. For most globulars, that means about  10 billion years ago. They have earned the right to look old and red. Yet, blue stragglers are bluer and more luminous — and hence more massive — than they should be after ten billion years of stellar evolution. Blue stars are hot and consume their fuel quickly, so if these stars had formed about ten billion years ago, then they should have fizzled out long ago. How did they survive?

Currently, there are two main theories that might describe how blue stragglers came about. The first suggests that stars collide and mergel which would transform them into hotter more massive objects.

The other describes a transfer of material between two companion stars.  Neither theory has been proved, but that’s why astronomers want to observe more about these young-looking stellar oldsters.

The basic idea behind both of these options is that the stars were not born as big as we see them today, but that they received an injection of extra material at some point during their lifetimes and this then gave them a new lease of life.

This brilliant ball of stars lies in the southern constellation of Ara (The Altar). It can be easily seen in a small telescope. It was first spotted in 1826 by the Scottish astronomer James Dunlop using a 22-centimeter telescope in Australia. The image shows this cluster in all its starry glory, complete with oldsters passing as young beauties.

 

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We Come From the Stars

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This is Our Home Galaxy, and a Couple of Neighbors

As the Milky Way rises over the horizon at the European Southern Observatory, its companion galaxies also come into view. The Large Magellanic Cloud (LMC) at far left lies about 160,000 light-years away, while the Small Magellanic Cloud (SMC, above and to the right of the LMC) lies about 200,000 light-years away. New simulations show that the LMC stole stars from the SMC when the two galaxies collided 300 million years ago. Microlensing events that have been observed are due to LMC stars passing in front of a stream of stars pulled from the SMC.
Credit: ESO/Y. Beletsky

When you look out at the night sky, you’re looking at our ancient home. Yes, Earth is our current home. But, in the grand scheme of things, the galaxy — and all the elements that make it are also our home.  The elements that make up our bodies, our planets, and our star all were either created in the Big Bang (hydrogen, for example), or inside other stars (carbon, oxygen, nitrogen, etc.).  Multiple generations of stars have lived and died in the galaxy, and we are the resulting “star stuff”.

But, there’s more than star stuff out there.  There are mysterious things that may tell astronomers more about types of matter in the cosmos and distribution of that matter throughout the universe.

Astronomers have been studying one of those two irregular-looking clouds of stars that appear just below our galaxy in this image to understand a category of objects called MACHO (Massive Compact Halo Objects). These were thought to be things about the mass of a star that were so faint they couldn’t be easily detected. Surveys of this region of our galactic neighborhood have been underway to see if MACHOs could be part of that mysterious collection of “stuff” called “dark matter” that seems to be an incredibly important part of the universe.

In order for MACHOs to make up dark matter, they must be very faint. To even decide if they’re “there”, astronomers looked for a phenomenon known as microlensing. During a microlensing event, a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten. If a MACHO does this, then they’d know a little bit more about the object.

By studying the LMC, astronomers hoped to see MACHOs within the Milky Way lensing distant LMC stars. The number of microlensing events observed by various teams was smaller than needed to account for dark matter, but much higher than expected from the known population of stars in the Milky Way. This left the origin of the observed events a puzzle and the existence of MACHOs as exotic objects a possibility.

Instead of MACHOs, a trail of stars removed from the SMC could well be responsible for the microlensing events. How do astronomers know this? They’ve done computer simulations showing that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC. Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from our point of view.

Although the evidence for the trail of lensed stars is persuasive, they haven’t been directly observed yet. That will take time, since these could be faint. A number of teams are searching for the signatures of these stars within a bridge of gas that connects the Magellanic Clouds. The computer models used to simulate the trail will point the way for astronomers to find the other “stuff” that makes up the galaxies… and intergalactic space.