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What Are Stellar-Mass and Intermediate-Mass Black Holes?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Artist concept of a black hole taking matter from a companion star.

Artist concept of a black hole taking matter from a companion star.

It may be because of the difficulty in describing black holes that we hold such a fascination with them. They are objects with zero volume and infinite mass, which defy all our conventional ideas about everyday life. Yet perhaps as equally intriguing as their description are the different types of black holes that exist.

Stellar-Mass Black Holes

These are the smallest type of black holes known currently and most form from what is known as a supernova, or the violent explosive death of a star. Currently, two types of supernova are thought to result with a black hole.

A Type II supernova occurs with what we call a massive star, whose mass exceeds 8 solar masses and does not exceed 50 solar masses (a solar mass being the mass of the sun). In the Type II scenario, this massive star has fused so much of its fuel (initially hydrogen but slowly progressing through the heavier elements) through nuclear fusion that it has an iron core, which cannot undergo fusion.

Because of this lack of fusion, degeneracy pressure (an upward force that arises from electron motion during fusion) decreases. Normally, degeneracy pressure and the force of gravity balance out, allowing a star to exist. Gravity pulls in while the pressure pushes outward.

Once an iron core increases to what we call the Chandrasekhar Limit (about 1.44 solar masses), it no longer has sufficient degeneracy pressure to counteract gravity and begins to condense. The iron core cannot be fused, and it is compacted until it blows. This explosion destroys the star and in its wake will be a neutron star if between 8-25 solar masses and a black hole if greater than 25 (Seeds 200, 217).

A Type Ib supernova is essentially the same as the Type II, but with a few subtle differences. In this case, the massive star has a companion star that strips away at the outer hydrogen layer. The massive star will still go supernova because of a loss of degeneracy pressure from the iron core and create a black hole given that it has 25 or more solar masses (217).

A key structure of all black holes is the Schwarzschild radius, or the closest you can get to a black hole before you reach a point of no return and are sucked into it. Nothing, not even light, can escape from its grasp. So how can we know of stellar-mass black holes if they emit no light for us to see?

Turns out the best way to find one is to look for x-ray emissions coming from a binary system, or a pair of objects orbiting a common center of gravity. Usually this involves a companion star whose outer layer gets sucked into the black hole and forms an accretion disk that spins around the black hole.

As it falls closer and closer to the Schwarzschild radius, the material gets spun to such energetic levels that it emits x-rays. If such emissions are found in a binary system, then the companion object to the star is most likely a black hole.

These systems are known as ultra luminous X-ray sources, or ULXs. Most theories say that when the companion object is a black hole it should be young but recent work by the Chandra Space Telescope shows that some may be very old.

When looking at a ULX in galaxy M83 it noticed that the source preceding the flare was red, indicating an older star. Since most models show that the star and black hole form together then the black hole must be old too, for most red stars are older than blue stars (NASA).

To find the mass of all black holes, we look at how long it and its companion object take to complete a full orbit. Using what we know of the mass of the companion object based off its luminosity and composition, Kepler’s Third Law (period of one orbit squared equals the average distance from the orbiting point cubed), and equating the force of gravity to the force of circular motion, we can find the mass of the black hole.

The GRB Swift witnessed.

The GRB Swift witnessed.

Recently, a black hole birth was seen. The Swift Observatory witnessed a gamma ray burst (GRB), a high energy event associated with a supernova. The GRB took place 3 billion light years away and lasted for about 50 milliseconds. Since most GRB last about 10 seconds, scientists suspect this one was the result of a collision between neutron stars. Regardless of the source of the GRB, the result is a black hole (Stone 14).

Though we cannot confirm this yet, it is possible that no black hole is ever fully developed. Because of the high gravity associated with black holes, time slows down as a consequence of relativity. Therefore, time at the center of the singularity may stop, therefore preventing a black hole from fully forming (Berman 30).

Intermediate-Mass Black Holes

Until recently, these were a hypothetical class of black holes whose mass is hundreds of solar masses. But observations from the Whirlpool Galaxy led to some speculative evidence for their existence. Typically, black holes that have a companion object form an accretion disk that can reach up to tens of millions of degrees.

However, confirmed black holes in the whirlpool have accretion discs that are less than 4 million degrees Celsius. This could mean that a bigger cloud of gas and dust is surrounding the more massive black hole, spreading it out and thus lowering its temperature. These intermediate black holes (IMBH) could have formed from smaller black hole mergers or from supernova of extra-massive stars. (Kunzig 40).

The first confirmed IMBH is HLX-1, found in 2009 and weighing in at 500 solar masses.

Not long after that, another one was found in galaxy M82. Named M82 X-1 (it being the first X-ray object seen), it is 12 million light years and has 400 times the mass of the sun. It was only found after Dheerraj Pasham (from the University of Maryland) looked at 6 years of X-ray data, but as far as how it formed remains a mystery.

Perhaps even more intriguing is the possibility of IMBH's being a stepping stone from stellar-mass black holes and supermassive black holes. Chandra and VLBI looked at object NGC 2276-3c, 100 million light years away, in the X-ray and radio spectrums. They found that 3c is about 50,000 solar masses and has jets similar to supermassive black holes which also inhibit stellar growth (Scoles, Chandra).

M-82 X-1.

M-82 X-1.

It was not until HXL-1 was found that a new theory for where these black holes came from developed. According to a March 1st Astronomical Journal study, this object is a hyper luminous x-ray source on the perimeter of ESO 243-49, a galaxy 290 million light years away. Near it is a young blue star, hinting at a recent formation (for these die fast).

Yet black holes are by nature older objects, forming typically after a massive stars burns through its lower elements. Mathiew Servillal (from the Harvard-Smithsonian Center for Astrophysics in Cambridge) thinks that HXL is actually from a dwarf galaxy that collided with ESO. In fact, he feels HXL was that dwarf galaxy's central black hole.

As the collision occurred, gases around HXL would be compressed, causing star formation and thus a possible young blue star to be in proximity to it. Based on the age of that companion, such a collision likely occurred about 200 million years ago. And because the discovery of HXL relied on data from the companion, maybe more IMBHs can be found using this technique (Andrews).

Another promising candidate is CO-0.40-0.22*, which is located in the molecular cloud its named after near the center of the galaxy. Signals from ALMA and XMM-Newton found by a team led by Tomoharu Oka (Keio University) were similar to other supermassive black holes but the brightness was off and implied 0.22* was 500 times less massive, clocking in at roughly 100,000 solar masses.

Another good piece of evidence was the speed of objects inside the cloud, with many reaching near-relativistic speeds based on the Doppler shifts the particles underwent. This can only be achieved if a high-gravity object resided in the cloud to accelerate the objects.

If 0.22* is indeed an intermediate black hole, it likely did not form in the gas cloud but was inside a dwarf galaxy that the Milky Way ate long ago, based off models that indicate a black hole is 0.1 percent the size of its host galaxy (Klesman, Timmer).

Works Cited

Andrews, Bill. "Medium Black Hole Once a Dwarf Galaxy's Heart." Astronomy Jun. 2012: 20. Print.

Berman, Bob. “A Twisted Anniversary.” Discover May 2005: 30. Print.

Chandra. "Chandra finds intriguing member of black hole family tree." Kalmbach Publishing Co., 27 Feb. 2015. Web. 07 Mar. 2015.

Klesman, Alison. "Astronomers Find the Best Evidence Yet for a Midsized Black Hole." Kalmbach Publishing Co., 08 Sept. 2017. Web. 30 Nov. 2017.

Kunzig, Robert. “X-Ray Visions.” Discover Feb. 2005: 40. Print.

NASA. "Chandra Sees Remarkable Outburst From Old Black Hole." Kalmbach Publishing Co, May 01, 2012. Web. Oct. 25 2014.

Scoles, Sarah. "Medium Size Black Hole Is Just Right." Discover Nov. 2015: 16. Print.

Seeds, Michael A. Horizons: Exploring the Universe. Belmont, CA: Thomson Brooks/Cole, 2008. 200, 217. Print

Stone, Alex.“Black-Hole Birth Seen.” Discover Aug. 2005: 14. Print.

Timmer, John. "Our Galaxy's Second Biggest Black Hole May Be 'Lurking' in a Gas Cloud." Conte Nast., 06 Sept. 2017. Web. 04 Dec. 2017.

Questions & Answers

Question: Will a black hole explode at the end of it's life?

Answer: Current understanding of black holes points to a no, because instead they should evaporate into nothingness! Yes, the final moments will be an outflow of particles but hardly an explosion as we understand it.

© 2013 Leonard Kelley


David Braun 4 En on March 23, 2020:

How do black holes change shape and do they expand or shrink.

David Braun on March 04, 2020:


David on March 03, 2020:

Thank you for this awesome passage I learned a lot

david on March 03, 2020:

What will happen if two black holes collide into each other?

david on March 03, 2020:

black holes are interesting wow i love this

Leonard Kelley (author) on June 03, 2018:

Thank you Ayandeep Maji!

Ayandeep Maji on June 03, 2018:

It is always interesting to see what the author means by different types of black holes. One can go by size, or one can go by inclusion or not of spin and electrical charge.

Nicely done.

Leonard Kelley (author) on July 23, 2015:

Thankfully it is far away. But we do owe it our existence!

Reality Check on July 23, 2015:

Absolutely love space related articles. The universe is truly unfathomable! And, of course, terrifying. A supermassive black hole at the centre of galaxies...?! Eeek! Great hub.

Leonard Kelley (author) on March 12, 2015:

Definitely many data metrics for sure.

Blackspaniel1 on March 12, 2015:

It is always interesting to see what the author means by different types of black holes. One can go by size, or one can go by inclusion or not of spin and electrical charge.

Nicely done.

Leonard Kelley (author) on February 28, 2013:

Thanks sparkster!

Marc Hubs from United Kingdom on February 28, 2013:

Great information and well written article. This is some fascinating stuff.