Astronomy: Stars, Black Holes, and Nebulae

The moment you are born, you begin the countdown to your death. Grim, and applies to almost everything. On average, we will only live for about 79 years. During this relatively short period, we study, work, and try to find our place in the world. Many humans worry about saving for retirement, ailments that come with old age, and what happens after we die. Luckily, we do not have to worry about eventual combustion that would scatter our remains across the universe.

Every star in space will eventually “die” in one way or another. But first, they must be born. Interstellar Medium (ISM) is the material that fills the space between the stars. ISM can be gas (hydrogen, helium) or dust (heavier elements, rocks). Ninety-nine percent of ISM is in a gaseous form, while one percent is dust. The ISM gas has an “average density of about 1 atom per cubic centimeter…[while] the air we breathe has a density of approximately 30,000,000,000,000,000,000 molecules per cubic centimeter” (http://www-ssg.

sr.unh.edu/ism/what1.html, pg 1). Consequently, in space, there is a low density but huge volume of ISM because the matter is spread out across vast distances. Nebulae are clouds of ISM that stars are created from. When the ISM begins to collapse under its own weight, it begins to form a star. The star’s life cycle is dependent on the amount of matter found in the nebula. The larger the star’s mass, the shorter its lifespan. This is because larger stars burn through their fuel much faster than smaller ones.

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Massive stars may only last for a few hundred thousand years, while their smaller counterparts could last for billions.

Over time, gravity pulls and spins the hydrogen gas in the nebula. As the gas spins faster, the collisions of the hydrogen atoms cause the temperature to rise. The nebula contracts and instability with gravity causes fragmentation. The fragments will keep breaking up and will eventually form tens or hundreds of stars from this one nebula. This process can take thousands of years. Now, it becomes a protostar. If the protostar does not have enough mass, it will become a brown dwarf. To be considered a brown dwarf, your mass is must in between 13 times and 80 times the mass of Jupiter. These are failed stars and hydrogen fusion is unable to occur in their cores. There are about six stars for every brown dwarf in our universe (https://www.jpl.nasa.gov/video/details.php?id=1089, pg 1).

If the protostar does have sufficient mass, when the temperature reaches 15 million degrees Celsius, nuclear fusion occurs in the core. To be considered a star, you must have core nuclear fusion and a mass greater than 80 times the mass of Jupiter, or a mass greater than 8% the mass of the Sun. Nuclear fusion is the combination of several small nuclei to form a single heavier nucleus. There is a release or absorption of energy depending on the masses involved in the fusion. The cloud begins to contract, glow and stabilize. Now, it is a main sequence star. It can remain in this stage for millions or billions of years, depending on its size. It takes about 40 to 50 million years to get to this level. Currently, our Sun is in this stage.

In our universe, about nine out of ten stars are main sequence stars (https://www.space.com/22437-main-sequence-stars.html, pg 1). In their core, hydrogen is converted into helium through nuclear fusion. This is called core hydrogen burning. A star begins its demise when the hydrogen supply of the core dwindles and the star is no longer generating heat through nuclear fusion. The core becomes unstable and contracts, making the outer layer of the star expand. The outer layer is still mostly hydrogen and starts to cool and glow red as it expands. This star is now classified as a red giant. All stars follow the same path until the red giant stage. Depending on their size, the star will take different paths to their ends.

For a low-mass star, like our Sun (864,400 miles across), the path to the end is relatively calm. The helium atoms will fuse to form carbon atoms in the core at 20 million degrees Celsius. The remaining hydrogen gas is used to form a ring around the core called the planetary nebula. At this point, the star is highly unstable, and begins to pulsate. A red giant starts to die when all the helium atoms have fused into carbon atoms. When gravity causes the star’s matter to collapse inward and compact, it becomes a white dwarf. The white dwarf is extremely dense and shines with white light, hence, the name. For example, one teaspoon of their matter would weigh as much as an elephant on earth. Since nuclear reactions do not occur in white dwarfs, they are reliant on their thermal storage of energy for all heat and light they emit. When it cools, it can become a black dwarf, which are invisible. Currently, there are no known black dwarfs.

For a high-mass star, the end is much more violent. Massive stars are at least 3 times the size of our Sun. Until the temperature reaches 600 million degrees Celsius, gravity causes helium atoms to fuse into carbon atoms. After the star reaches this temperature, carbon atoms create heavy elements such as oxygen and nitrogen. The star continues this until it starts to develop iron, the most stable and compact of all the elements, and fusion stops. It is harder to create heavier atoms through the fusing of iron and requires an input of energy. Since the energy is not being radiated from the core, the star begins gravitational collapse. The iron atoms are crushed together, the repulsive forces between the nuclei overcomes the force of gravity. This results in a shock wave, which is seen by humans as a Type II Supernova.

This massive explosion can light up the sky for weeks, and can reach 1 billion degrees Celsius. Due to this high temperature, new elements may appear in the creation of a new nebula after the supernova. The shock wave sends remnants of the supernova into space at speeds of 9,000 to 25,000 miles per second. A supernova explosion will also cause the elements and debris of the star to spread across the universe, where it could be reused or become another planet or star. We, humans, are also made from the stars. The carbon, nitrogen, oxygen, and other heavy elements in our bodies were created from these supernovae (https://www.livescience.com/32828-humans-really-made-stars.html, pg 1). A Type I Supernova differs from a Type II because it is the detonation of a white dwarf. The white dwarf was accumulating gas from a companion star which caused its eventual detonation.

After the supernova, the remains of the star will take one of two paths depending on their size. If the remnant of the explosion is about 1.4 to 3 times the size of our Sun, it will become a neutron star. This neutron star can also be known as a pulsar, which rotates and generates regular pulses of radiation. If the remnant of the explosion is more than 3 times the size of our Sun, it will be swallowed by gravity and become a black hole. This occurs because the force of gravity overpowers the nuclear forces keeping the protons and neutrons from combining. Black holes swallow any matter and energy that comes near it.

Nonetheless, sometimes, a star skips the supernova explosion and directly turns into a black hole. N6946-BH, A star 25 times the size of our Sun, was projected to have a supernova explosion. Instead, it petered out and became a black hole. In 2009, N6946-BH began to shine weakly, and by 2015, it seemed to have disappeared. NASA’s Hubble and Spitzer space telescopes were both used to confirm the disappearance and to see if there were any infrared rays still present. These tests came back affirming that N6946-BH had vanished. “As many as 30 percent of such stars, it seems, may quietly collapse into black holes –no supernova required” (https://www.jpl.nasa.gov/news/news.php?feature=6858, pg 1).

This percentage was derived from the NGC 6946 “Fireworks” Galaxy, where there was one failed supernova (N6946-BH) and six normal supernovae within a seven-year period. It is still unknown why N6946-BH became a black hole. Krzysztof Stanek, professor of astronomy at Ohio State university, “suspect[s] it’s much easier to make a very massive black hole if there is no supernova’ (Stanek, 1). A supernova blasts the star apart, sending pieces to every corner of the universe. Skipping that step, the star would still retain its mass and therefore, possess more gravity. After hundreds of years studying the stars, humans still cannot exactly predict everything that could occur.

Supernovae do not occur frequently in the Milky Way galaxy, about once every 50 years. The first supernova recorded was back in 185 A.D. by Chinese astronomers. RCW 86 was documented as a “’guest star’ that remained in the sky for eight months” (https://www.nasa.gov/multimedia/imagegallery/image_feature_2173.html, pg 1). Astronomers who studied the x-ray and infrared data concluded that it was a Type I supernova. RCW 86 was about 8,000 light-years away.

With modern technology, we can detect supernovas happening in the furthest corners of our known universe. The furthest we have detected is 10.5 billion light-years away. This star exploded billions of years ago, when the universe was only around 3.45 billion years old (current age of the universe = 13.8 billion years old). DES16C2nm was detected in August 2016. It is also a super luminous supernova (SLSN), which is the rarest and most luminous class of supernovae. This class was established in 2008. SLSN are believed to be formed when material falls onto the neutron stars, one of the densest objects in the universe. It is assumed that the supernova produces a magnetar (neutron star) at its core. “…[T]he light from the super luminous supernova…matches…models of the amount of energy that magnetars emit as they spin” (http://www.astronomy.com/news/2018/02/astronomers-discover-the-most-distant-supernova-ever-detected, pg 1). When the energy hits the ejected materials winds, it dramatically lightens up what we see. Despite being virtually on the other side of the universe, we could see this supernova because of its super luminous properties.

Supernovae indicate the end of the line for the star it explodes. But what about its effects on planet Earth? Supernovae can destroy any nearby planets within about 30 light years distance. Supernovae release cosmic rays and iron-60 (60Fe) when they occur. Cosmic rays are high energy atom fragments that carry electric charges. Iron-60 is a radioactive isotope that can cause radioactive iron rain on Earth. ISM and our ozone layer would have a role in how much cosmic rays and iron-60 pass through space and our atmosphere respectively. Firstly, the ISM would slow down the cosmic rays and iron-60 for hundreds of thousands of years. The higher energy particles would arrive much sooner than the lower energy particles. Secondly, our ozone layer is what protects us from ultraviolet radiation from the sun. The high energy particles would pass through the stratosphere and deposit their energy under the ozone layer, while the low energy particles would deposit their energy in the stratosphere. The low energy particles would have a greater negative effect, depleting the ozone significantly. This would expose life on Earth to further damage incurred from the Sun and asteroids.

The life cycle of a star takes millions or billions of years to go from start to finish. Stars first develop from nebulae, or patches of interstellar space matter (ISM). In due time, this cloud will become a protostar. If this protostar does not have enough mass, it fails and becomes a brown dwarf, where hydrogen fusion does not and cannot occur. If it does possess enough mass nuclear fusion will occur in its core, and will eventually turn into a main sequence star. Our Sun is a main sequence star. When core hydrogen burning stops in the core of a main sequence star, the core becomes unstable and contracts.

The outer layer of the star expands and now, it’s a red giant. At this point, size matters. Low mass stars will form a planetary nebula, eventually turn into a white dwarf and then a black dwarf. High mass stars will have a gravitational collapse that will lead to a type II supernova. After a supernova, a star can become a neutron star and then a pulsar, or a black hole. Hundreds of years ago, we discovered and recorded the first supernova we saw. With modern technology, we can see supernovae billions of light years away. Supernovae seldom often occur in our vicinity, and if they did, we would be at risk of destruction. Fortunately, the sun is not predicted to go out with a bang with a supernova, but will become a white dwarf. All of us will be long gone by then.