Question: How do you calculate the critical mass for a star to become a black hole? I’m an undergraduate Math major, and this is my senior thesis and I really have no idea where to start. – Lauren
Answer: An earlier post to this blog on “Properties of Stars which Result in Black Holes” provides a general overview to this question. Since the exact mass of an object like a star that must ultimately become a black hole is a function of its radius, there isn’t an exact mass above which that object must collapse to a black hole. Said another way, any object which collapses to the point where its radius is less than a certain limit must ultimately become a black hole. This radius is called the Schwarzschild radius (Rs), and it is given by the following equation:
Rs = 2MG/c^2
where M is the mass of the object, G is the gravitational constant, and c is the speed of light. If you plug in values for the constants G and c and use solar masses for M and km for Rs, this equation reduces to the following rather simple form:
Rs = 2.95*M(solar masses) km
So, for a star with the same mass as our Sun, the Schwarzschild radius is about 3 km, or about 2 miles. In general, stars with final masses in the range 2 to 3 solar masses are believed to ultimately collapse to a black hole.
Question: I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why? — Doug
Answer: As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate. Kind of “beauty in the eye of the beholder” problem. Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:
Question: Will dark energy eventually tear apart black holes, especially supermassive black holes? – Reinaldo
Answer: Let’s start with a basic understanding what what Dark Energy is thought to be. It is a hypothetical form of energy which is thought to pervade the universe. It has been proposed as the source of the measured acceleration of the expansion of our universe. As a physical entity it is really quite “thin”, having a density of about 10^(-29) grams per cubic centimeter, so its affect of things like planets, stars, humans, and black holes is exceedingly small. But, because it is thought to fill the universe its affect on the expansion of the universe is very important. So, to answer your question, Dark Energy does not affect the properties of black holes, so in fact cannot disrupt them in any way.
Question: I have just been watching a very interesting programme on the television about scientists who are studying the super massive black hole in the centre of our Milky Way galaxy. It was stated that that black holes are formed when huge stars die and collapse in on themselves. Taking the super massive black hole in the centre of the Milky Way galaxy as an example, is it possible for scientists to run a retrospective analysis to ascertain the magnitude of the star which died and caused the black hole to form. – David
Answer: Yes. Computer modelling of the evolution of very massive stars have shown us that a star with a mass greater than 20 times the mass of our Sun may ultimately become a black hole. Once the star runs out of fuel to drive its nuclear engine gravity takes over, compressing the star which ultimately collapses. This collapse results in a supernova, which expels the outer parts of the star but leaves the core to collapse even more. If the core has a mass which is greater than 2.5 times the mass of our Sun, gravity takes over and causes the core to collapse to form a black hole.
Questions: Please may I ask some questions regarding black holes: – David
- Have black holes always existed ever since the “Big Bang”?
- What happens if two black holes were to collide, do they destroy one another or merge into an even larger black hole? Have scientist ever witnessed such an event?
- What is the life span of a black hole and what happens to it in the end?
- There are theoretical predictions that suggest that black holes may have existed even *before* the Big Bang (during what some have called the “Big Crunch”, or the contractive event that preceded the Big Bang). These would be “primordial black holes”. The “normal” process for producing a Black Hole involves the collapse of a very massive (larger than 50 times the mass of our Sun) star as it exhausts its nuclear fuel. So, more standard theories would place the first Black Holes as forming only after the first massive stars formed, which happened when the Universe was about 200 million years old.
- The collision and eventual merger of two Black Holes would indeed be a violent event. It would send out intense energy that would potentially be detected as a burst of gamma rays or gravitational waves. Such an event has never been identified, though.
- It is not clear how Black Holes evolve, and if they have a lifespan. One theory predicts that a Black Hole will slowly radiate away mass via “virtual particles”. Note, though, that the existence of these virtual particles is in theory only.
Question: I would like to know what would show that a black hole
was moving farther away from you? My specific wonders would be how light around the hole would act, how its shape would change, or if the black hole would start to reflect less and less light? – Billy
Answer: Remember that you cannot really “see” a black hole. All you really see is stuff being gobbled-up by the black hole, which emit very high energy light just as they are being sucked into the black hole. So, if you noticed that the high energy light that is emitted by these objects was getting “redder”, or being emitted at a longer and longer wavelength, that would be a sign that the black hole was moving at a more rapid pace away from you. If a black hole is embedded within an environment where its supply of matter was decreasing, then you would see less and less high energy light coming from the region around the black hole as this supply dwindled. There is a nice description of what a black hole looks like (with pictures!) on the Universe Today description regarding just this question.
Question: I think I know what is in black holes [and] why black holes are black, because they swallow everything, even light? Well my theory is that nothing but what the black hole sucked in is in there and that everything is just compacted together and light particles are some how disrupted by the intense gravity. I just thought it might be thrown out there since people have so many theories. I’m only 14 so some of it might not make sense but I thought it might work. thanks for listening. – Vinnie
Answer: You are right! The simple interpretation of a black hole is that it just plain eats everything, even light. Nothing can escape. The details of this process and how it affects the properties of a black hole are very complicated, but your basic description is spot-on!
Question: Can a black hole be swallowed by a super massive black hole? And if it could, how would we be able to see it? – Shayna
Answer: Yes. If such an event occurred, and we were looking in the place to be able to see it, we would likely see a burst of high-energy radiation, which we can see as intense x-ray emission from such an event. An example of such a merger of black holes that might happen in the future has been observed in the galaxy merger system NGC3393 by the Chandra x-ray observatory. I hope that this answers your question.
Question: I know that black holes are very dense and their gravitational pull is so strong no one really knows what they look like because they don’t come out alive. I have read that there might be one in the center of galaxies well why doesn’t it suck the Milky way galaxy in? – Lily
Answer: It is a matter of relative scale. Galaxies are quite large; many kiloparsecs in size, while black holes are quite small, only a few millionths of a parsec (a parsec is about 206265 Astronomical Units, where an Astronomical Unit is the distance from the Earth to the Sun). A single Black Hole, even one at the center of our Milky Way galaxy, is just too small to eat an entire galaxy.
Question: What should the critical mass of a black hole be in order for the star to be torn apart? How does one find this critical mass? – Safar
Answer: The general answer to your question depends upon a number of factors related to the star/black hole system, but we can look at one example to get an idea as to what size of black hole we need to tear a star apart. Such an event has in fact been observed earlier this year (see the Scientific American article describing this event). Spectroscopic measurements of a flare from a star/black hole system as the star was being devoured was used to determine what chemical species exist in the material being eaten by the Black Hole. This indicated that the material being eaten was from the core of a Red Giant star. Also, by measuring this system while the Black Hole dined on the Red Giant star over more than a year scientists were able to determine the mass of the Black Hole, which they found needed to be about three million times the mass of our Sun.