Expert: Jesse Martinez - 2/22/2010

Question
QUESTION: i was reading recently about the red supergiant antares is 700 times larger than the sun to understand clearer what can you compare a penny with to equal this size? next, i was also reading about the black hole that was measured by austin astronomers,this black hole is 18 billion times the size of the sun,what can the penny be compared with to show the size difference?the black hole's weight is approximately 18 billion solar masses,how many pounds is that?thanks

ANSWER: First question: Place a penny at the base of a four story building and imagine a circle as tall as the building. Or imagine a marble compared to a large hot air balloon. That is the scale of the difference in size between two objects where one is 700 times wider than the other.

Second question: Black holes are dimensionless. This means they have no size. They are points. What is observable is the accretion disk, which is roughly the area of its gravitational influence. We see the matter caught in this area.

Third question: 18 billion solar masses is is about 79,200,000,000,000,000,000,000,000,000,000,000,000,000 lbs.
Whew, that is one big honking number!

P.S. Even though your answer is dated Feb. 5th, I only received it on Feb 18th. I was not putting you off. I somehow only just received it. Hope the answers help.

---------- FOLLOW-UP ----------

QUESTION: thanks for answering my first set of questions, i have another set. so you are saying that what the astronomers are describing to be 18 billion times the size of the sun is the black holes accretion disc? can you please explain what a accretion disc is and also what can the penny or marble be compared with now or what is 18 billion times larger than a penny or marble?thanks

Answer
An accretion disk is an area in which matter is collected by a strong gravitational force like a black hole. The size of the accretion disk varies greatly. Other objects besides black holes have them, including super massive stars and protostars and neutron stars. The size of an accretion disk is not fixed. The figure of 18 billion times the size of the sun is really meaningless.

This subject is extremely complex. You have points of matter that have no volume, but have a specific mass. Black holes cannot be of infinite weight, but mathematically they can be of infinite density or undefined density depending on how you do the math. The "size" of a black hole is likewise another meaningless term. The best answer is that it is the same as the area it is affecting. See this site for information about systems affected by black holes.


http://burro.astr.cwru.edu/stu/stars_blackhole.html