Black holes are a popular topic, one that I’m asked about quite often.
By far the most commonly asked question is: “If black holes are black, and light can’t escape from them, how do we see them?” That’s a great question!
A black hole is an object that packs a very large mass in a very small volume. They are a theoretical predication of general relativity proposed by Karl Schwarzschild, Subrahmanyan Chandrasekhar and others. The gravitational pull of a black hole is so strong that nothing can escape from it.
Black holes are formed when a massive star collapses at the end of its life, and they grow by consuming gas or merging with other massive objects. It is this growth that betrays their presence. Objects rarely fall directly into a black hole because everything in the universe has its own independent motion.
Instead, matter tends to spiral into a black hole, much like water going down a drain. If enough matter is spiraling into a black hole at the same time, that matter will heat up due to frictional forces. As the infalling matter heats up, it begins to emit electromagnetic radiation. The area around a black hole where this happens is called an accretion disc.
Depending on the size of the black hole and the amount of infalling matter, the accretion disc can form a large magnetic field and beam out massive amounts of radiation through jets at the poles of this field.
The electromagnetic radiation generated by these accretion discs is one of the primary ways we detect black holes. The first black hole we discovered, called Cygnus X-1, was found because of the X-rays emitted by superheated gas in the accretion disc of the black hole.
Most galaxies harbor a supermassive black hole at their center. The term “supermassive” is not used lightly. The Cygnus X-1 black hole is about 15 times more massive than our sun, with a radius of only 45 kilometers. The black hole at the center of the galaxy Centaurus A is about 55 million times more massive than the sun.
When black holes grow to this size, and they have sufficient matter falling into them, they can have accretion discs that outshine entire galaxies, with jets that stretch millions of light years. These types of black holes are called active galactic nuclei or quasars.
The Morning Sky
Mars shines in the south east before dawn. The red planet rises around 1:30 a.m. and is getting a little brighter every day. Venus has returned to the morning sky. Watch as it climbs higher in the east each morning.
The Evening Sky
Look to the west after nightfall to see the beautiful crescent moon just to the left of the constellation Leo on the evening of the 26th. You can visualize the moon’s orbit around the Earth as it moves about 13 degrees to the east against the background stars each night. More of the moon becomes illuminated each night also. The amount of light falling on the lunar nearside increases from 35% on the evening of the 26th to nearly 95% on the evening of the July 2.
Dan Price is a NASA/JPL Solar System Ambassador and informal educator. He leads the Night Sky Tours at Josephine Sculpture Park. Visit his Starpointe Astronomy YouTube channel on or after Saturday to learn more about black holes. Have a question about astronomy or space science? Send an email to email@example.com and it might be featured in a future column.