Podcaster: Morgan Rehnberg

Title:   Monthly News Roundup –  It’s a Small World

Link :

Dwarf stars:
Stellar-mass black holes:
Mars ice:
Planet vortex:

Description: In this episode of the Monthly News Roundup, we look at tiny stars and small black holes.  Trenches on Mars might not come from water, and planet-forming might be possible after all!

Bio: Morgan Rehnberg is a graduate student in astrophysics and planetary science at the University of Colorado – Boulder.  When not studying the rings of Saturn, he develops software to help search for asteroids that might hit the Earth.  He blogs and podcasts about astronomy and space science at

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You’re listening to the 365 Days of Astronomy Podcast for June 30th, 2013. I’m Morgan Rehnberg and this is the Monthly News Roundup. This episodes was produced by Cosmic Chatter and recorded June 26th from Boulder, Colorado.


We start this month with the smallest stars in the cosmos. There is an amazing variety of stars out there, from tiny ones only a fraction the size of our Sun to behemoths that are thousands of times larger. But just how small can they get? New research revealed this month suggests that it’s pretty small – less than ten percent the radius the Sun, and only a thousandth as bright.

Stars are defined as bodies which use the process of nuclear fusion to generate from hydrogen the energy which sustains them. In a sequence called the CNO cycle, hydrogen atoms are combined into helium, carbon, nitrogen, and oxygen. Each step releases energy which we then see as light emitted from the star. This process, however, requires enormous heat, which is generated by enormous pressure. If a body is too small, it can’t generate enough heat to get the process started. Objects right on the lower edge can’t fuse hydrogen, but they can fuse it’s heavier sibling deuterium. We call these bodies brown dwarfs because of how dim they appear.

Just where the dividing line between normal star and brown dwarf lies has been debated for decades. A commonly acceptable value today is that stars must be at least eight percent the mass of our Sun. That defines how heavy the star must be, but what does that mean for it’s size? That’s where this new research comes into play.

Physics often behaves strangely under extreme conditions and the center of a star is among the most extreme in Nature. One of the more counterintuitive results in this area is that adding material to a star can actually make it smaller. If it’s dense enough, the extra mass compresses the star down further than it would have built it up in size. This means that the smallest stars can actually be smaller than the largest planets. Although a planet like Jupiter might be larger that a tiny star, it still weighs far less because, in the end, it’s the mass that matters.

It’s likely that these dwarfs are the most common type of star in the galaxy, but they’re so dim that we’ll probably never get to see most.


Let’s move from small stars to small black holes. Recent observations of the nearby galaxy Andromeda have revealed a host of new stellar-mass black holes. These tiny objects are called stellar-mass because they typically have masses only a few times that of the Sun. This compares to their supermassive cousins, which can can weigh the equivalent of millions of stars.

Black holes are the final stage of life for the largest stars. As stars age, they use up their hydrogen fuel, which limits their ability to produce new energy. Without this energy to support its incredible mass, the star quickly collapses. As the material crunches together, it explodes like a bomb. The outer layers of the star are thrown off as a supernova. Gravity in the core of the star, however, has become too strong to overwhelm and the core collapses to a black hole. At this point, not even light can escape.

So, if not even light can escape a black hole, how can we detect them? It turns out that we can only do so indirectly. When material falls into a black hole, it bumps into other stuff on the way in. These collisions heat up the material, often to millions of degrees. Just like on Earth, hot things glow, and we can observe the light emitted in these regions to detect a black hole. Because the temperatures are so hot, stuff falling in often emits x-rays instead of visible light.

While we’ve found supermassive black holes at the center of most nearby galaxies, smaller stellar-mass ones are far more difficult to observe. Most of the ones we know about are in our own Milky Way, so finding a bunch in Andromeda is a great opportunity to expand our understanding of this curious phenomenon.


It seems like every day we find some new evidence that points towards a wet history for Mars. Whether it’s aqueous minerals, ancient lake beds or actual subsurface water ice, our orbiters and rovers continue to strengthen the case. In science, though, it’s important to consider alternative explanations for our observations. Consider, for example, the case of the trough-like gullies which have been observed in various places on Mars.

If you subscribe to the notion of a historically-wet Mars, then these gullies have an easy explanation – flowing water. Streams and rivers cover the Earth and we likewise observe a preponderance of gullies. But rivers didn’t create all the dramatic landforms on Earth – glaciers played their part as well. Flowing ice carved the Great Lakes and scraped clean the Canadian shield – what could it have done on Mars?

First off, it seems rather unlikely that large flows of water ice ever persisted on Mars. The ice likely at work here would have been made of carbon dioxide – commonly called dry ice. Carbon dioxide ice can still be found on the surface of the Red Planet today: it makes up the seasonally changing polar ice caps.

So, eons later, how can we differentiate between ice flows and streams? Evidence here on Earth is key. When water flows, it picks up material and deposits it along the way. This is why rivers and canals must be continually dredged to allow the passage of ships. Glaciers, on the other hand, push material in front of them and deposit it all at once.

Observations from the Mars Reconnaissance Orbiter reveal that many gullies appear free of deposited material. Others, of course, do appear to be created by flowing water. Perhaps unsurprisingly, the more we learn about the martian surface, the more complex the picture becomes.


Finally this month, we tackle one of the most important open questions in astronomy. In the wake of the Kepler mission and other planet-hunting efforts, we’ve found perhaps thousands of unique star systems. But how did they form and what leads to the incredible variety that we’ve already glimpsed?

Some parts of the process are already pretty clear. Planets form out of the same disk of dust and gas which originally gave birth to the central star. Tumbling around as it orbits this young star, bits of gas and dust stick together and slowly increase in size. Eventually they get big enough for gravity to take over and bind them into large rocky objects. For planets like Earth, the story ends here. Other planets use their newfound mass to suck up nearby pockets of gas – the because the Jupiters and Saturns of the Universe.

This picture might seem pretty cut and dry, but I’ve glossed over the critical step. Once these small clumps of dust reach a few centimeters in size, they start to feel drag from the gas around them. Just like when throwing a ball, this drag acts to slow down the clump. In orbit, however, slowing down is a death sentence. As the clump slows, it spirals inward and eventually hits the star. Therein lies the rub. If this happens to all small clumps, how do we ever get to the stage of forming planets? We just don’t know.

Observations published this month from the brand-new Atacama Large Millimeter Array in Chile hint at one possible solution. These observations seem to show that a large vortex has formed around a planet in one part of the disk. Dust trapped in this vortex might be able to grow beyond the centimeter size without falling victim to a collision with the star.

Unfortunately, however, we still have a chicken-and-egg problem. If a planet is necessary to form the vortex, how do we get the first planet in a system? Clearly this isn’t the only mechanism for overcoming the size problem. It’s emblematic of the overall troubles, however. No theory has yet to propose a complete solution and, with more observations flooding in every day, the problem is getting even harder.


Thanks for listening to this episode of the Monthly News Roundup. For more astronomy news and commentary, visiting or follow @cosmic_chatter on Twitter. You can contact me with comments and corrections via email at See you in July!


End of podcast:

365 Days of Astronomy
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