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Questions Show: Stellar Roche Limits, Seeing Black Holes, and Water on Mars

Cataclysmic Variable

Cataclysmic Variable

This week we find out when stars get torn apart from gravity, how we can see supermassive black holes, how liquid water could have existed on Mars in the past, and much more.


  • Stellar Roche Limits, Seeing Black Holes, and Water on Mars
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    Meet the Astronomy Cast team Live in Long Beach California!

    If you live near Long Beach or are there for the meeting, meet Pamela, Fraser, and the Astronomy Cast LIVE team at a “Blogger Meet-up” on Wed. January 7 at the Rock Bottom Brewery from 6pm – 9pm.

    Is there a “stellar” Roche Limit for binary stars?

    Why are black holes seen as bright areas in astronomical images?

    How could there have been water on Mars in the past?

    Are there lots of jobs out there for propective astronomers?

    What evidence do we have that the Universe was small, hot and dense just before the Big Bang?

    What legal issues are there for astronomy and space exploration?

    Why don’t planets “twinkle”?

    Transcript: Stellar Roche Limits, Seeing Black Holes, and Water on Mars

    Download the transcript

    Fraser Cain: Happy New Year Pamela! Let’s get on with this week’s show. This week we find out when stars get torn apart from gravity, how we can see super massive black holes and how liquid water could have existed on Mars in the ancient past and much, much more.

    Now let’s get on with the first question. This question comes from James Arsenaut: “I understand that in a planet-moon system there’s a certain orbital level that the moon must maintain to avoid being torn apart by tidal forces and that’s the Roche Limit. My question is: Is there a similar point in a binary star system?”

    A Stellar Roche Limit if you will. So, I think we did two shows about tidal forces and we said that if a moon orbiting a planet gets low enough the gravity on the front side is different enough from the gravity on the back side of the moon that it starts to get torn apart.

    Instead of being a nice round moon, it sort of turns into a ring and then the ring crashes into the planet. So, same deal, could you have a star orbiting another star and it get so close that one star gets torn apart?

    Dr. Pamela Gay: Yes. This is actually what cataclysmic variables are. You have a white dwarf or a neutron star in some rather violent cases that go beyond cataclysmic variables. You can even have a black hole, just a stellar mass tiny one orbiting some sort of other star. It could be a main sequence star, a giant star, or a regular non-compact object.

    If it gets close enough such that the material at the surface of the star is gravitationally balanced between the pull of the star it’s supposed to be attached to and the star that it is orbiting around it will actually start to pull out and you end up with a teardrop shaped star. As we say, its Roche lobe is filled. I

    f you start to go past that point of balance you can actually see materials stream off of the surface of one star and form a disk around the compact object that’s nearby. Over time the materials in that accretion disk will in some cases if the disk gets too dense it will actually ignite.

    You can have the material actually flow onto the surface of the star increasing the size of the compact object the white dwarf. This can cause supernova eruptions.

    There are all sorts of neat systems. You can also just get two run of the mill stars without the compact object involved where if they orbit close enough and close enough together they can actually end up with merging envelopes. You have two stars that basically share one shell of gas around them.

    Fraser: Then it just depends on how close they are what the speed is that they’re going and whatever gravity differences you’re dealing with. Whether they both get torn apart and share each other or one gets torn apart or what happens.

    Seeing a star with a Roche lobe, that would be pretty interesting. It would be like as you said like a teardrop shaped star orbiting a compact object. That would look so weird.

    Pamela: And like I said, these are out there. We just can’t resolve them because they’re all too far away.

    Fraser: Right but we can tell that that’s what’s going on.

    Pamela: We build them in computers and we plot out what is the gravitational pull at all different areas and the places that have the same gravitational pull those get filled up basically with material.

    Fraser: So what’s good for planets and moons is the same for stars. If you get too close, you get torn apart.

    Alright, let’s move on. This question comes from Mohammed: “If at the center of most galaxies is a super massive black hole that not even light can escape, then why is it that when you see a Hubble image of a black hole the center is highly luminous?”

    I think we’ve had this question a little bit before, but we’ll keep answering it. [Laughter] It is, how can we see black holes?

    Pamela: I think what we’re dealing with here is when you look at the center of a galaxy what you end up seeing is this really bright region, the brightest part of the galaxy down in the core. In the case particularly of active galaxies where that super massive black hole is in the process of eating stuff, you can end up with tremendous amounts of light getting jettisoned sometimes in high-powered jets even that are visible at distances much greater than the size of the main disk itself.

    What is happening is as material falls into the super massive black hole it ends up building an accretion disk just like the ones we were talking about with cataclysmic variables. This disk of material can get as dense and as hot as the center of the star.

    It will undergo its own thermo-nuclear reactions and it essentially becomes just like the core of a star giving off its own light. So, what you’re seeing is the light from the accretion disk surrounding the super massive black hole.

    Fraser: Right, we’re not seeing the black hole itself, we’re seeing the material getting gobbled up that is around the black hole.

    Pamela: Now if you look at the images that have been taken of the core of our own Milky Way Galaxy where we can see stars zipping around a point of nothingness in the images.

    There we don’t see light coming from the super massive black hole because our super massive black hole is kind of boring. It’s not in the process of eating anything.

    Fraser: Then if the black hole isn’t eating anything, then it is invisible and that’s because nothing, not even light is escaping it.

    But if the black hole is in the process of consuming something then we see the accretion disk and we see the light pouring off of it.

    Pamela: Yeah.

    Fraser: We’re seeing the light from the disk not the black hole.

    Pamela: Exactly.

    Fraser: Right and that’s material waiting to die.

    Pamela: Very definitely. [Laughter]

    Fraser: Alright. Jack asks: “How could liquid water have existed in the past on Mars?” So today obviously Mars’ cold, dead, dry world…

    Pamela: No liquid.

    Fraser: No liquid, red rocks as far as the eye can see. On the poles there is water ice but it is locked in ice that’s probably been there millions if not billions of years with a nice layer of carbon dioxide that snows down every winter on top of it.

    Pamela: There is water vapor in the atmosphere and you do get new ice deposits, but you never get liquid.

    Fraser: You never get the liquid. If you did take a glass of water and put it out on the surface of Mars, what would happen to it?

    Pamela: [Laughter] it would freeze instantly.

    Fraser: Right, so that would be that. What if it was warm though? Mars can be warm. You can have ten degrees above zero.

    Pamela: Right so then you end up with pretty much it goes to vapor almost instantly. What’s happening is the atmospheric pressure on Mars is extremely low.

    Everything goes pretty much straight from ice to vapor without going to a liquid phase in-between unless it’s really, really salty and then it’s only temporarily at the surface.

    Fraser: So if I put a glass of water out on the surface it would just boil and in moments becomes vapor and emptied.

    Pamela: When the air temperature is above its freezing temp.

    Fraser: Right. Yeah that would be so weird to see.

    Pamela: It would be awesome.

    Fraser: Just, you know, room temperature, glass of water out would just boil away and then be gone.

    Pamela: Yeah. Now the thing is the way Mars is now isn’t the way it was in the past.

    Fraser: Aha.

    Pamela: Once upon a time Mars had a much thicker atmosphere so you had additional pressure on the water. You also had higher temperatures. By combining both higher temperatures and higher atmospheric pressures, you’re able to end up with water in the liquid phase.

    Remove both the temperature and pressure or either one of them radically enough and the possibility of liquid water goes away.

    Fraser: We cover this a bit in our Terraforming Mars episode. If you thicken the atmosphere on Mars you could get to the point again where temperatures would warm up and liquid water would be fine.

    The poles would melt and the temperatures would be fine. It’s just like an insane amount of air pressure they’re trying to bring back to the planet.

    Pamela: We’re not going to be pumping it from Earth to Mars. [Laughter]

    Fraser: It would just be an enormous amount of gas to return to the Earth. The problem is that all of the Oxygen is locked up in the rocks all over the Hydrogen that you require. Most of it has been blown away from the planet. You really don’t have a lot to work with to thicken the atmosphere.

    Pamela: Because Mars is so much smaller even some of the larger atoms very quickly get blown away by the solar wind. In the past when Mars went through heavy bombardment periods, you have comets whonking it and throwing their materials both into liquid on the surface and into greater atmospheric material, it created it but it was very temporary.

    If we were somehow able to take a chunk of the Kuiper belt and dump it onto Mars we might be able to reproduce some of those conditions. It would be temporary because again, those atoms & molecules would just blow in the wind.

    Fraser: So Mars probably had a similar formation to Earth, probably had a nice thick atmosphere in the beginning but because it doesn’t have the gravity, it couldn’t hold on to the atmosphere. The solar wind blew it all away and you’re left with Mars without really any thick atmosphere.

    Pamela: Exactly.

    Fraser: Alright, but you could return it again Jack. If you want to thicken up the atmosphere, feel free to re-make Mars and have some liquid water and go for a swim.

    Next question comes from Laura: “I don’t really know much about the everyday of a working astronomer. I also wanted to know what kind of job opportunities there are. Also, is it a field that is already over-populated making it hard to find a job or is it a field that is really in need of people to fill the gaps?”

    Now, we did a whole episode on how to get a career in astronomy so I think that’s going to solve most of the questions that Laura asks here. But, I guess quickly, are there a lot of jobs?

    Pamela: No. It’s actually we’re producing more people that have degrees than there are jobs available.

    Fraser: Uh-oh.

    Pamela: Yeah, so it’s a field where people go, they get PhDs even and then they end up being computer scientists or they end up working for the stock markets. The skills that you need in astronomy, the computer skills, the math skills, the ability to simulate things and to solve problems are skills that are useful in a variety of different fields across industry.

    The degree is half the pursuit. For a lot of people once you’ve gotten the degree, that other half of the lifelong career in astronomy, it’s just you can’t get there from here. There aren’t enough jobs.

    There are also alternative careers that you can have even if you don’t have the PhD in astronomy. For instance you Fraser, you don’t have the PhD, you have a great job in astronomy as a communicator.

    Fraser: Right. You gave me some examples of positions that open up. If you get like a faculty position opening up at a university you might have one position and have hundreds of people battling for it, right?

    Pamela: Right, that’s completely normal. That’s happened at my own institution. Just getting into graduate school you can often have 150 people competing for 5 positions.

    Fraser: The positions are there but you’ve got to be the best of the best of the best. Not only are you in a really hard math, science course at university, but you’ve got to be getting really high marks, right?

    Pamela: Yeah and the expectations are really quite amazing nowadays because astronomy is a field where people love it, are passionate about it and so universities are able to be extremely picky in who they accept.

    They are going to get this range of wonderful people so nowadays it’s not unusual for undergraduates to be expected to do research that leads to publications in science journals in order to get into graduate school. You’re dealing with people who are participating in research in their late teens, early twenties helping to understand new bits of the Universe.

    The Mars-H team for instance had undergraduates working on some of its cameras as the developers and the people out at mission control when it landed in helping to get the data off from the mission. It’s these people that get involved from day zero age, 18 straight out of high school in doing astronomy that are eventually ending up getting the faculty positions 20 years down the line. It does take that long to get there. But there are other avenues.

    There are planetariums, science centers, observatory positions. There’s everything from the person that operates the telescope all night to people who are out there setting up remote telescope systems to help amateurs observe around the world.

    Fraser: There are lots of opportunities for amateurs and just as a hobby. Astronomy is something that you can give a lot back. There are a lot of opportunities for hobbyists to contribute with working professionals.

    The weird thing that you mentioned is that you may be a professional astronomer but you’re having really difficult time gathering data. So you reach out to amateurs all the time to gather data for you. It’s a really weird thing.

    For the faculty positions that pay, there aren’t a lot of jobs. Yet the need for data is enormous. The opportunity for amateurs who know what they’re doing to help gather material is also enormous.

    But they just don’t get paid. [Laughter] They do get their names in journals and it can be another track to moving towards a faculty position. If you have the education and you can demonstrate that you have helped out in a lot of journals.

    Pamela: What’s really cool is I’ve seen a person go from doing the amateur astronomy to getting degrees with Swinburne Astronomy Online, who is actually one of our sponsors, to eventually ending up in faculty positions.

    It doesn’t happen very often, it’s fairly rare, but what is fairly common with people who come out of Swinburne Astronomy Online is that they end up at planetariums and science centers.

    One of the folks behind Globe at Night is someone who went the amateur astronomy route, the masters in astronomy online route and he’s actively involved in a major international collaboration to help protect our skies.

    Fraser: I think if you want to go the standard route, go to university, get your PhD, apply for positions, get on faculty, etc., it is a very competitive market and it is very difficult.

    You’ve got to really invest a lot of effort and be prepared for some big sacrifices in the rest of your life for not a lot of pay. [Laughter]

    Pamela: That’s true. That’s so true. [Laughter]

    Fraser: But if you’re willing to think creatively and if you’re willing to take some risks and sort of follow your own heart and not necessarily follow the beaten track, I think there’s a lot of opportunity.

    As you said, my background is in software but I was interested in this enough and just kept putting in the work and made the contacts and connections. Now I sort of feel like I’m a part of the industry and didn’t have to get that pesky PhD.

    Pamela: You saved yourself a whole lot of tears.

    Fraser: A whole lot of time and money, yeah. I think all of the opportunities are there. All of the directions are possible. If you really love it, then you’ve got to do it. If you don’t really love it and you just think like maybe this is a good career for me, it’s probably not a good career for you.

    Pamela: Go check out what the folks at AAVSO, at Unmanned Space Flight forum and at Galaxy Zoo forums are doing as they take their getting data one step further and start being scientists.

    Fraser: Right. We get lots of questions from the readers and this is one of the questions that we really love to get. We really love to help. If you want to help build a career in astronomy or you want to participate, you want some directions, feel free to send us an e-mail and we or Scott who answers a lot of our e-mail will try and put you in the right direction.

    It’s really important for us to help all of you if you are interested in astronomy to get involved, to make a difference, to participate and help move science forward and have a career that you find fascinating. It really sucks to be in a service job [Laughter] or even a career that you just don’t really enjoy. If you’re listening to AstronomyCast then I think you like this. You like space, you like astronomy and you probably thought about making it a career.

    There are a lot of opportunities out there and if we can help you sort of maneuver through and sort of figure out what your passions are then we’d love to help. So keep sending in your e-mails. We get them a lot and we are happy to get them.

    Let’s move on then. This question comes from Shanti: “What is the evidence to prove that the Universe was extremely small and extremely dense before the Big Bang?”

    Okay so, Big Bang, 13.7 billion years ago was the event that the Universe that we have today started. If we trace back all of the movements of the galaxy it appears that they all started out in one point billions of years ago.

    What evidence do we have that the Universe was actually small and extremely dense even at the time of the Big Bang?

    Pamela: We can’t actually say anything about before the Big Bang. As far as we know we can’t get any evidence of anything before time started and time started at the moment of the Big Bang.

    Fraser: Right.

    Pamela: But, what we do know is roughly what the density is now. What we do know is what the density was at the moment the Cosmic Microwave Background Radiation was released. As we work backwards and look at things like what are the patterns in the Cosmic Microwave Background?

    We’re able to determine that there was a higher density time. There was a hotter time and this implies that basically we live in an expanding Universe. Every bit of evidence that we’ve looked at points towards this one consistent: take the Universe and as you roll time backwards it gets smaller, denser and hotter.

    At moment zero, everything was approximately a singularity. I say approximately because there are people that say wave functions. There are people who say point. Exactly what it was at the moment of the Big Bang, that’s something for theorists to argue.

    Fraser: Right so in terms of analogy we’ve got two cars driving away from each other. Right now we look at them and the two cars are say 500 meters apart and moving at 100 kilometers an hour each one. We also have video evidence of the two cars being, I don’t know, 10 meters apart and moving at 30 kilometers an hour.

    We don’t have any evidence of before that, but we’re sort of doing the math and saying well if they’re this far apart now and they were that far apart then, they had to be touching. We could even calculate when they would be touching. This would be the same thing, right? Physicists use the math. They calculate what the Universe must have looked like.

    Pamela: That’s how we got where we are.

    Fraser: We have no way to actually see it, right?

    Pamela: No. We actually can’t observe anything from before the release of the Cosmic Microwave Background Radiation. I’d encourage you to go out and we actually did an entire episode on Cosmic Microwave Background so go listen to that and find out how we know about that part of the Universe.

    Really a lot of our understanding of how our cosmology has evolved comes from studying the Cosmic Microwave Background.

    Fraser: The wording on this question – you picked it out – which was before the Big Bang. So if the Universe was a small compact object at the moment of the Big Bang or shortly after the Big Bang began does that not tell us anything about what it was beforehand?

    Pamela: No.

    Fraser: And why not?

    Pamela: There are the wave function people where you can sort of imagine the Universe is some probability function, some fluctuation of waves. At the moment of the Big Bang, all those waves lined up just right, formed a huge spikey peak and our Universe emerged from it.

    There are all sorts of other which I understand less wave function ways of looking at it. There are the well nothing existed before the Big Bang. We can’t even go there from here, just stop thinking about it. I tend to fall into the stop thinking about it because math breaks..

    Fraser: Really?

    Pamela: Yeah, you can’t get logical before the Big Bang so why try and speculate? Then there are the multi-verse people that say that basically we have a landscape of constantly emerging bubbles of Universe where the constants, the values, perhaps even the charge on the electron varies from Universe to Universe.

    We just happen to be one grain of sand in this cosmic landscape and that particular grain of sand blew up rather radically.

    Fraser: I think we need to do a show on before the Big Bang. Obviously disclaim it and say…

    Pamela: [Laughter] this is not a facts-based journey.

    Fraser: There are no facts here – nothing. But it’s neat to think about the ideas and the theories that people have thought of so far. They’re pretty cool. Right now there is no way to know what came before the Big Bang. It’s a nonsense question.

    Pamela: Exactly.

    Fraser: But at least we know using math that the Universe was teeny tiny and hot early on.

    Pamela: We leave the science fiction writers to everything else.

    Fraser: Perfect. Here’s another career one, this comes from Kevin Kelly: “I recently graduated from college with an English degree and I plan on attending law school in the fall of 2009. Astronomy is a hobby of mine and I was hoping to pursue a legal career towards issues related to astronomy and space exploration. So my question is what legal issues affect astronomers? Is there such a thing as space law?”

    Pamela: YES! This is actually really cool. Like we’re going to the Moon – who owns it? We don’t know.

    Fraser: Me! [Laughter]

    Pamela: We’re going to have to sort this out through international treaties. It’s going to begin to get very complicated as we have all sorts of different nations working collaboratively but then who owns the building?

    Just with the international space station, that’s a huge multi-national agreement. Who owns what? How much crew do you get? What is the allocation of resources? Who pays for what? It gets very complicated where you’re combining basically contract law, international law, property rights and you’re redefining everything.

    Basically it’s the Pope dividing the planet between Portugal and England. We’re going to be dividing up the Moon, Mars, trying to figure out mineral rights for asteroids. All of these things should be happening in our lifetime. The legal implications are huge. It would be a wonderfully fun place to go.

    Fraser: The European Space Agency has a whole Center for Space Law. I know that there is a space law blog. So there are space lawyers right now. Absolutely, it is absolutely a viable career choice.

    As we start to do more and more with space tourism there are so many legal issues with it. With what are the safety issues?

    Pamela: That’s a liability waiver I don’t want to write.

    Fraser: No, I know. Restricted air space – what does that mean when you’re flying into orbit? You know flying over other people’s territory. There are so many things that are going to happen.

    When you think about it right most of the Universe is not on Earth [Laughter] so that’s where most of the law would be, right?

    I know there is definitely a serious career path. People who consider themselves to be space lawyers and there are centers and institutions that specialize in it.

    Pamela: That is by far the best thing you’ve ever said: “Most of the Universe isn’t on the planet Earth.”

    Fraser: Right, so whatever you think about that stuff. You think about alien biology. Way more work out in space than there is here on Earth. We just have to – it’s more of a transportation issue.

    Pamela: We just need to find it.

    Fraser: Great question so DO IT! I love it.

    This comes from Santosh from Singapore: “When I was chatting with my friend, he astonished me with a question which I have not been able to answer, here it goes. If twinkling of stars is caused by the Earth’s atmosphere, why are the planets not twinkling?”

    This is what people have been using to distinguish a planet from a star. Is that true, do stars twinkle and planets don’t?

    Pamela: Only when the sky is only kind of mediocre. If you have an absolutely perfect, perfect idealized will never happen if I’m in an observatory because I attract bad weather, type of night. Nothing twinkles.

    The twinkling is actually caused by atmospheric effects where you end up with pockets of hot and cold air moving past each other and the light bends as it goes from regions of that have different temperatures. This constantly changing bending of the light as the pockets of different temperatures moves causes the path that the light takes from the top of the atmosphere to our eye to vary slightly. We refer to this as seeing when we’re taking images.

    As the path changes the star, which is a point of light on the sky, moves slightly. This slight motion, the slight actually changing of the shape of the light, we refer to it as twinkling.

    A planet to our eye is still mostly a point but once you start looking at it with a telescope you can end up with stars focused down to less than half of an arc second where an arc second is the width of a piece of hair held out at arm’s length. If you take and imagine a tenth of the width of a piece of hair held out at arm’s length, that’s the size of a star on the sky due to atmospheric stuff.

    A planet can be tens of arc seconds across. That difference in size actually means that well yes – each individual photon’s path varies. The numbers of photons that vary towards the center of the planet you’re not going to see them twinkle. The ones in the center of the planet that vary towards the outskirts of the planet but not past the edge you’re not going to see their twinkling.

    So you only end up with shimmering around the edge of the planet unless the atmosphere is really, really, really noisy in which case everything shimmers. Everything twinkles.

    Fraser: So because all of the light that we see from a really bright star is coming from effectively a point source. Even in the largest telescopes any little bit of motion of bending from the atmosphere makes the star move back and forth.

    But because the planet is a disk we don’t see it. Hubble would have a terrible time knowing what a planet is and what a star is because it sees no twinkling.

    Pamela: Yeah, but it’s able to resolve the size of all of these things. It even sees Pluto as a disk. It has other advantages.

    Fraser: Absolutely. That was great. I think that wraps up this week’s question show.

    2 Responses to “Questions Show: Stellar Roche Limits, Seeing Black Holes, and Water on Mars”

    1. Stewart says:

      lol, you messed up on “water on mars.” It will boil and freeze at the same time. Basic physics: Enthalpy and phase diagrams. See youtube video:

      This also has an impact on your space suit podcast. The side of an astronaut away from the sun won’t flash freeze from lack of solar radiation, but from evaporation. A person could easily withstand the radiative heat transfer to a 0K black body for hours. Simple Stefan–Boltzmann law (T^4). A man could survive quite awhile at ~ -20C (-8F). That is roughly 1/2 the radiative heat transfer compared to 32C (98.6F) as compared with 0K. It is the cooling caused by evaporation which would flash freeze the backside.

      Btw: Please don’t mind the criticism, you guys are wonderful and I have learned a lot from you.

    2. Sapjes says:

      This beats an episode about the top 10 things Jesus would like to do in January!

      Thank you!

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