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One of the more difficult ideas to grasp in astronomy is the sheer scale of the cosmos. Even as a professional in the field it’s much easier to focus on numbers and data than really step back and appreciate scale. Every now and then there is an image that forces you contemplate scale, such as the Hubble Ultra Deep Field. Now NASA and ESA have released a new image of the Andromeda galaxy. It is the most detailed image of Andromeda ever made,and you can check it out in this video.

The total image has about 1.5 billion pixels, and shows over 100 million stars across 40,000 light years. In the image you can see the brilliant swirls of Andromeda’s spiral arms, but when you zoom in the dusty swirls resolve into individual stars. A vast sea of stars.

First photograph of the Great Andromeda Nebula by Isaac Roberts, 1899

Andromeda is relatively close by cosmic standards. It’s only 2.5 million light years away, which is why we can resolve the galaxy in such detail.  But this shows only the brighter stars of Andromeda. It’s estimated that Andromeda contains about a trillion stars, most of which are too small and dim to resolve in this image. We’re still only seeing a fraction of the stars in this galaxy.

As you look on this image, keep in mind that this is only part of a nearby galaxy. Our own galaxy would look similar, as would many others. It’s estimated that there are 100 billion galaxies in the visible universe. That’s more than 10 galaxies for every man, woman and child on Earth. Multiply this image by ten, and for all of it there is only one single human being. The same is true for every person on the planet.

Images such as these often give a feeling of smallness. Within the vast cosmos we are merely dust upon dust. But to me it demonstrates just how rare and wondrous each of us are. We can look on an image such as this, see the spirals of stars, and we can understand the cosmos.

The post My God, It’s Full of Stars appeared first on One Universe at a Time.

Science news has been a buzz about a whole posse of new black holes discovered in the constellation of Andromeda. The reason for this is not that these have suddenly been discovered, but rather that a new paper on results from Chandra is being published in the Astrophysical Journal last year, and the press releases have been published.

Chandra is an x-ray space telescope, which makes it particularly good at finding what are known as x-ray binaries. If you remember my post on microquasars earlier this week, you might recall that an x-ray binary is a binary system of a regular star and a compact object that is either a neutron star or a black hole. When these two are relatively close, material from the regular star is swept off and captured by the compact companion. As the material swirls it is heated so much that it gives off x-rays. The result are localized x-ray sources.

Chandra made 152 observations of Andromeda over 13 years, and identified 26 candidate solar mass black holes. Confirming these is a challenge for two reasons. The first is that you have to confirm the x-ray source is actually in the Andromeda galaxy, and not further away and behind Andromeda. This is done by looking at the average brightness and the overall brightness of the x-ray sources. X-ray binaries all have somewhat similar brightnesses, and since we know the distance to Andromeda we know basically how bright they should appear. But it could be possible that a very bright supermassive black hole much farther away would have a similar apparent brightness, so astronomers have to look at the variation in brightness as well. Supermassive black holes are millions of times larger than a solar-mass black hole. Because they are bigger, the variation in brightness is much slower. So by looking at the brightness variation we can confirm that these 26 candidates are actually in Andromeda.

The next challenge is to distinguish x-ray binaries that are black holes from those that are neutron stars. This is a bit less certain, because they appear rather similar. Because black holes are more compact than neutron stars, the x-rays generated by infalling matter tends to more intense for black holes than for neutron stars. But there isn’t a hard and fast line between one and the other. Of the 26 candidates, 12 were categorized as strong, meaning they are almost certainly black holes, and 14 were considered plausible, meaning they are likely black holes, but it’s possible they are neutron stars. Further observations should resolve that uncertainty.

One of the interesting aspects about this study is that seven of the black holes lie within 1000 light years of the supermassive black hole at Andromeda’s center. You can see these in the figure above. This is a larger number than we’ve found in our own galaxy, and may tell us about the evolution of galaxies like ours. Another interesting observation is that 8 of the candidates are in globular clusters. We’ve long suspected that black holes could be found in globular clusters, but we haven’t observed any such black holes in our galaxy. We now know that globular cluster black holes exist.

One last thing, some of the popular news sources (cough- Fox News -cough) are stating that a trove of black holes have been found “near Earth”. Granted, 2.5 million light years is a fairly small distance on cosmic scales, but it isn’t what you might typically consider “near.”

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The image below is the first complete x-ray survey of the Andromeda galaxy. The Andromeda galaxy (also known as M31) is about 2.5 million light years away. It is a spiral galaxy very similar to our own Milky way, so surveys of Andromeda are very useful in understanding galaxies like ours.

Credit: H. Stiele, et al

The survey observed almost 2000 x-ray sources, and from these a few patterns can be seen. For example, the survey identified dozens of supernova remnants, and these tend to be located within the star forming regions of Andromeda. This would imply that these are remnants of very large stars.

Other sources were identified as x-ray binaries. These are binary systems consisting of a regular star and a compact partner such as a neutron star or black hole. Gas from the star is captured by the compact partner, which heats it tremendously, resulting in x-rays.

More than half of x-ray sources couldn’t be identified as either given the limits of the survey, so it will take further observations to determine what they might be. Still, this the most detailed x-ray survey we’ve made of a galaxy beyond our own.

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