professor moore

Dakota Brown writes,
“What is the expected life span of a stellar mass black hole?”

When a massive star ends its life after burning all its fuel via nuclear fusion, the central region collapses into a black hole that has the mass close to that of the Sun. The collapse releases a huge amount of energy – a supernova. A black hole is the name given to something that is so massive that even light cannot escape from its interior.
We don’t know a great deal about the internal structure of a black hole. Gravity has overcome all the electro-nuclear forces that we know hold atoms together, but this doesn’t mean that there is some as yet unknown physical process that prevents the collapse to a singularity.
So, how long does the black hole last? If Stephen Hawking is correct, black holes will slowly radiate away mass through a quantum mechanical processes involving ‘virtual particles’. A stellar mass black hole is expected to exist for a long long time, much longer than the age of our universe. Its difficult to comprehend how long; ~10^67 years compared to the age of the universe which is ~10^11 years.
However, virtual particles are a mathematical construct used to explain the properties of matter and the forces of nature on the smallest scales. This overall theory is very successful, one of the best tested theories that we have in science in fact, but the reality of virtual particles remains to be understood. They have never been directly ‘seen’ in an experiment since they can’t exist for a long enough period of time. If virtual particles are a reality then Hawking may be correct. But until we have a better understanding of quantum mechanics, or a detection of Hawking’s radiation, we can’t be sure that a black hole couldn’t last for eternity.
Cheers!
Ben

John A. writes,
“Finding all those exoplanets got me thinking – how many exoplanets with stable orbits (over millions of years) could there be in the habitable zone of a star? Expanding this question: Does the size/the type of the star change the size of the habitable zone and thus the number of exoplanets?”

The habitable zone around a star is the region at which a planet can orbit without its surface water freezing or boiling away. It’s quite a narrow zone – if our Earth were 10% closer to the Sun our oceans would boil away, if it were about 50% further away the oceans would freeze. However, this doesn’t mean that life could not evolve in other places, such as within Enceladus, the moon of Saturn which lies well outside the habitable zone. The icy surface of Enceladus hosts an interior ocean of warm water that is prevented from freezing by internal radioactivity and gravitational squeezing by Saturn.

Our computer simulations show that almost ever star should have about one planet in the habitable zone. For the details see here: http://adsabs.harvard.edu/abs/2010Icar..207..517M
There is rarely more than one planet in this region since they would be orbiting quite close together and perturb each other over long timescales. Most of the ~50 billion stars in our galaxy have a mass not too different from our Sun, so there should be plenty of life in our Galaxy.

Hal M. asks,
“I was wondering how close would an object approximately 1 km in length have to be before it could be detected by our most powerful/far-reaching telescope?”

If you are asking, “how far away could you detect a 1km object?”, this depends on its brightness, or how many photons it emits or reflects. In principle, you can detect a 1km object at the edge of our visible universe if it is bright enough, although it would appear as a point in space. The edge of the universe is not really an edge, its just the distance that light has traveled in the age of the universe. Since space is stretching over this time (13.7 billion years) the furthest you can possibly detect an object is about 50 billion light years.

If you are asking, “how far away can you resolve a 1km object?”, then take the resolving power of the Hubble Space Telescope as an example. It has a resolution of about 0.1 arcseconds, so it could resolve and measure a 1 km object that is about 4 million km away. It could resolve a 100 meter object on the surface of the Moon. Technically, the answer also depends on the wavelength of the light that you are observing.