ASTR 1514 1st Edition Lecture 31 Black HolesChapter 13.4-13.61.) Stellar Evolution has three possible endpoints- White Dwarf Star – the end-point of a medium mass star, White dwarfs are less than 1.4 solar masses- Neutron Stars – The end-point of massive stars. Neutron stars are between 1.4 and 3 solar masses.- Black Holes – The end point of very massive stars, with massive cores greater than 3 solar masses. - If the mass of a neutron star exceeds 3 solar masses it will collapse to a black hole- Not even light can escape the gravitational pull of a black hole- Can form directly from type II supernova (if massive enough) or from accretion by a neutron star in a binary system. 2.) Understanding Black holes- A black hole has mass, but it has no size. These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- It is a singularity, meaning you can approach a black hole close enough that the escape velocity is equal to the speed of light - Event Horizon: The radius at which the escape velocity is the speed of light- In principle, Black holes can have any mass- Ex: If we could make the Earth smaller and smaller the escape velocity would get larger and larger- Typically a black hole is born at the end point of a massive star of 10 solarmasses, therefore they have an event horizon about 3km in the radius. - If you are far from a black hole you will not notice and difference in spaceor time around you. - Near a Black hole- Einstein considered gravity in a different way, he proposed that massesdistort space and time within and around them, resulting in a curved space-time. 3.) General Relativity- The theory of general relativity was tested by observing the apparent shift of a starnear the sun during a solar eclipse. - The sun is large enough to bend space time a very little, near it’s surface.- The orbit of Mercury precesses, the axis shifts due to the bending of space near the sun. 4.) Falling into a black hole- Gravity is stronger as we get closer to the black hole, so someone falling in the black hole will fall faster and faster the closer they get to it. - Light is bent around the black hole, so we would experience some unusual opticaleffects as we got close. *Gravitational Lensing- Light is bent around the black hole, so you can see behind it- More light rays are bent in your direction, so the object is brighter than if the black hole were not there- Gravitational lensing can be used to view distant galaxies. The cluster of galaxies acts as the lens. - If you are falling into the black hole feet first, your feet will feel a stronger gravitational force than your head. - Stretching you- If you survive the stretching then after 65 days you will cross the event horizon- You would not notice anything, since the event horizon is not a hard surface or boundary.5.) Someone observing the black hole from Earth- An observer watching you fall into a black hole will witness you falling slower and slower as you get closer to the black hole- From his perspective you never fall into the black hole, you just stop- Black holes were initially called “frozen star” because of this. 6.) Time Dilation- Clocks will run more slowly when gravity is strong- The effect of time dilation is stronger when you get closer to a black hole- We can observe time dilation from light emerging from near a black hole- Light has a frequency, which is like a clock. - Decreasing the frequency due to time dilation increases the wavelength- Called gravitational redshift. - “Tired Light” – Light loses energy fighting to get away from the black hole. 7.) Your Effect on the Black Hole- A black hole is characterized by two numbers: Mass and Spin.- You would simply add to its mass if you fell in8.) Seeing Black Holes- Black Holes are very dark- We see them when gas from a stellar companion falls onto the black hole.- The gas heats up, due to conversion of gravitational energy to thermal energy. - See black holes through X-ray binary systems. - Neutron Stars can also be seen in binary systems- They have nearly as strong gravity as black holes, so they also emit X-RaysAccreting neutron stars and accreting black holes are sources of X-Ray