Yesterday I talked about how we can measure the size and shape of our galaxy even though much of it is obscured by gas and dust. As I pointed out, we can only observe a small fraction of the stars in the Milky Way. But then how do we know how many stars there are in our galaxy? The short answer is we don’t, but we can get an idea by measuring the mass of our galaxy.

Typically the way the mass of our galaxy is measured is to measure the speed of stars and other objects in the Milky Way. This gives us a mass of about 100 – 400 billion solar masses. Not all of that is due to stars, but it gives us a basic idea of the number of stars in our galaxy. But recently a more accurate measure has been determined, and it was done by observing globular clusters. Globular clusters are dense, spherical cluster of stars. Their shape comes from the fact that they are clustered under their own gravity. They are not located in the plane of our galaxy, but instead are found in all directions in a kind of halo around our galaxy. They were first used by Harlow Shapley to determine the basic structure of our galaxy in the early 1900s.

The thing about globular clusters is that they aren’t completely gravitationally bound. Through various gravitational interactions, a few stars escape a globular cluster at a regular basis (a process called evaporation). As a result, these clusters leave a trail of stars behind as they slowly orbit our galaxy. In this recent work the team looked at these trails and found that they had variations of density within them. This means the rate of evaporation varied over time. When they analyzed these variations, they found a pattern, and this pattern is dependent upon the overall mass of our galaxy. Basically that means the evaporation is affected not only by gravitational interactions within the globular cluster, but also interactions between the cluster and our galaxy. So the team used this data to determine the mass of our galaxy. What they found was that within a radius of 60,000 light years  the mass is about 210 billion solar masses.

It should be emphasized that this is only the mass of our galaxy within a 120,000 light year diameter, which is about the diameter of the visible galaxy. Beyond this region our galaxy is dominated by dark matter, which makes up most of the total mass of our galaxy. So we can say that the Milky Way has roughly 200 billion stars.

Paper: Andreas H. W. Küpper et al. Globular Cluster Streams as Galactic High-Precision Scales – The Poster Child Palomar 5. ApJ 803 80 doi:10.1088/0004-637X/803/2/80 (2015)

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Yesterday I mentioned the possibility that a mid-sized black hole might explain why the Trapezium cluster is gravitationally bound. If such an black hole were discovered it would be a really big deal.

We know that black holes exist, but they seem to come in only two sizes: stellar and supermassive. Stellar black holes are formed when large stars reach the end of their life and their core collapses. They typically mass less than 20 solar masses. Supermassive black holes lie at the heart of galaxies. They form during a galaxy’s youth and can be a million solar masses or more.

Between these two extremes should be intermediate mass black holes (IMBHs). These would be hundreds or thousands of solar masses. Too large to have formed from a single star, but not a supermassive heart of a galaxy. An intermediate black hole could form from a stellar mass black hole by accreting gas, dust or nearby stars. They might form by stellar collisions within a dense star cluster, or perhaps they could form during the early universe when gas and dust were more dense.

We don’t know of any reason why intermediate black holes couldn’t form, but so far we haven’t observed any. We’ve discovered lots of black holes, but so far they only come in small or large. We do however have some indirect evidence that intermediate mass black holes exist.

One example can be seen in the image. It is a composite image of x-ray (red) and optical (blue/white) observations. The ULX box indicates an Ultra Luminous X-ray source. This x-ray source varies in brightness about every two hours. This is interesting because the rate at which an x-ray source varies is determined by the mass of its source. The x-rays come from the accretion disk of the black hole, since different portions of the accretion disk can’t interact faster than light can travel between those regions, larger accretion disks vary more slowly than smaller accretion disks.

With a period of two hours, that would put the the black hole at about 10,000 solar masses. That would make it an inbetweener. This is, of course, assuming it is actually a black hole. The most likely explanation is that this is a mid-size black hole, but it will take more detailed observations of the dynamics to rule out alternatives such as a very dense star cluster.

So if the Trapezium cluster really does have a mid-sized black hole, that would be pretty cool.

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Here’s a headline for you:  Physicists find black holes in globular star clusters, upsetting 40 years of theory  This story (and others with similar titles) has been making the rounds of science fans.  Time for a bit of fact checking.

There has been an interesting black hole discovery recently.  It was announced in the Astrophysical Journal last month.  What the team found was a stellar mass black hole in the globular cluster M62.  The globular cluster M62 is in our galaxy, so this marks the first black hole found in one of our globular clusters.  Finding a black hole in a Milky Way globular cluster is a good result because it is relatively close.  M62 is only about 22,000 light years away, so it will allow us to better study a black hole in a globular cluster.

It is not, however the first black hole found in a globular cluster.

Several black holes have been found in globular clusters.  In fact, this Summer I wrote about the discovery of a slew of black holes in the Andromeda galaxy, including 8 in globular clusters.  So finding a black hole in a globular cluster is not entirely uncommon.

What makes this discovery interesting is that it wasn’t found by x-ray observations, which is how most black holes are discovered.  Instead this one was found by radio observations.  As the black hole streams off jets of material, the jets can be radio-luminous.  In this particular case the team observed a radio luminous jet, indicating a black hole.  The authors are careful to refer to it as a candidate black hole, since there is a possibility that the radio signal is from another mechanism, or from something behind the globular cluster (called a background source).  Future observations will determine if that is the case, but right now it really does look like they’ve found a black hole.

So how does this upset 40 years of theory?  It doesn’t.

Globular clusters are tightly packed clusters of stars that orbit a galaxy.  We aren’t entirely sure how they form, but they typically contain old, low-metal (population II) stars, and this means they are themselves older objects.  Many globular clusters have also undergone what is known as core collapse, where close encounters between different stars in the cluster cause the more massive objects to gravitate toward the center of the cluster, while less massive objects are pushed to the outer regions.  This process is known as mass segregation.

The “40 years of theory” stems from a 1969 paper that discusses this mass segregation process.  One of the conclusions is that the most massive objects in the center (in this case black holes) would have close encounters that eject them from the cluster (a process called evaporation), leaving only 1 or 2 black holes.  Globular clusters that have undergone mass segregation, so the theory goes, would eject most, or perhaps all of their black holes.  But this isn’t the only theory about black holes in globular clusters.  Other models propose that mass segregation could lead to mergers, forming an intermediate mass black hole (a few hundred to a few thousand solar masses).  So far there are some observational hints that such a black hole may exist in globular clusters, but nothing definite.

So finding solar mass black holes in globular clusters runs counter to the idea of black hole “evaporation”, but it is not as if this discovery has radically overturned a long held truth.

Just to be clear, this new discovery is good work.  It just doesn’t merit the sensational headlines.

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