Jul 23, 2020 in Exploratory

Stars, Black Holes, and Galaxies


Stars, black holes, and galaxies are objects or systems that are found as part of the solar system or the universe. They were formed at different points in history and as a result of varying processes. The formation of the stars precedes that of the black holes owing to the fact that the latter emanate from the collapse of the former. On the other hand, galaxies are thought to have been formed alongside the universe following the “big bang”. In this regard, the formation of galaxies can be regarded as having taken place before the other two were formed. Also worth noting is that stars form part of galaxies. This essay will discuss the three objects and as will be noted, quite a lot is known about the three although they are still being studied by institutions and individual scientists.



The National Geographic Society (2015a) defines a star as a cosmic energy engine that emits light, heat, x-rays, ultraviolet rays, as well as other radiation forms. Stars are mostly comprised of gas, and a superheated matter made up of subatomic particles known as plasma. The most familiar star is the sun. It stands alone whereas three-quarter of all stars are found as a component of a binary system that contains two mutually orbiting stars. It is not known the number of stars that currently exists as the number keeps staggering. However, it is thought that the universe most likely hosts up to more than 100 billion stars contained in 100 billion galaxies. Interestingly, only about 3000 stars are seen using naked eyes on a clear dark night sky on earth.

The appearance of stars is also a subject that has been widely studied. Some stars have been found to stand out from the others, their brightness being an aspect of the amount of energy they produce, or their luminosity, as well as the distance from the earth form which they are located. Stars also tend to appear as differently colored owing to the fact that they have varying temperatures. Blue or white-colored stars are hot while orange or red hued stars have cooler temperatures. The size of stars also varies considerably. They are classified as ranging from dwarfs to super-giants with the latter having a radius bigger than the sun’s radius by up to a thousand times.

The primary building block of stars is hydrogen. The gas is found circling through space in nebulae or cosmic dust clouds. The clouds gradually condense and collapse as a result of gravity. As they decrease in size, nebulae tend to increase their spinning speed since the angular momentum is being conserved. For such a nascent star, temperature is raised by the building pressures. On the other hand, nuclear fusion often takes place when the core temperature of a developing star rises to approximately 15 million degree Celsius.

Stars start their lifecycle as protostars. Throughout their development, they increasingly accumulate mass obtained from the surrounding clouds and become what is referred to as main sequence stars. These are stars that exist in a nuclear fusion state and they produce energy for billions of years as they convert hydrogen to helium.

The evolution of stars takes billions of years. On reaching the end of their main sequence stage, they go through other existence states depending on such characteristics as their size. As they approach the end of their lifecycle, stars have most of their hydrogen already converted to helium. This helium goes down to the core raising the temperature of star and thereby causes an expansion in its outer shell. “Red giants” is the term used to denote these large swelling stars. This phase precedes another state in which the star sheds its outer layers and diminishes in size forming a dense body known as a white dwarf. It takes billions of years for white dwarfs to cool down upon which they become dark and stop producing any energy. The resulting stars are known as black dwarfs, but they have not yet been observed by scientists. Few stars skip this phase and instead, detonate in a bang as supernovae. The violent explosion causes the formation of a small core known as a neutron star. In other cases where the remnant is large enough, what is formed is known as a black hole, as will be seen later in this paper.

Black Holes

According to the National Aeronautics and Space Administration (NASA), a black hole refers to a location in space in which the force of gravity is so high that not even light can be able to get out. This strong force emanates from the fact that matter is found squeezed into an extremely small space, and it is usually the situation when a star is approaching its death. Since it is not possible for light to get out, humans are unable to see black holes, although specialized space telescopes can aid in finding them. The special tools in such telescopes are often in a position to see the manner in which the stars located too close to the black holes behave differently compared to their counterparts in other locations.

A brief history of black holes as explored by NASA indicates that the term black hole was coined in 1967 by a physicist based in Princeton by the name John Wheeler. Theorist Einstein also predicted the existence of black holes in his theory of general relativity that demonstrated that following the death of a massive star, a small-sized dense core is left behind. The theory’s equations also showed that if the mass of the core was larger than approximately thrice that of the sun, the gravitational force will overwhelm any other force and result in the formation of a black hole.

The size of black holes varies considerably with some being big and others small. Scientists are of the view that the smallest ones are the size of a single atom. Despite their extremely tiny size, these black holes have a mass that equals that of a large mountain. Here, it is important to note that the mass denotes the volume of the matter within an object. A different sized black hole is that known as stellar. It has a mass that is as high as 20 times that of the sun. It is thought that stellar black holes are numerous in the earth’s galaxy. The term “supermassive” is used when referring to the largest black holes. The mass of these black holes is higher than to that of 1 million suns brought together. It has already been established that all large galaxies feature a supermassive black hole located at the center. In the case of Earth’s galaxy, also known as Milky Way, the supermassive black hole is referred to as Sagittarius A and its mass is compared to that of 4 million suns. It is also projected that the Sagittarius A can fit within an extremely large ball capable of holding a number of million Earths.

The formation of the smallest black holes is thought to have taken place when the universe started. On their part, stellar “black” holes are formed as a result of the collapsing of very big stars, or when they fall upon themselves. This event results in formation of what is known as a supernova, as mentioned earlier. This is an explosion of a star in which part of it is blasted into space. In the case of supermassive black holes, scientists think that they were formed parallel to the formation of the galaxy in which they are found.

A common question with respect to black holes regards how scientists know of their existence if they are indeed black. Notably, scientists often detect the manner in which the strong gravity affects the stars and gas found around the black hole in question. It is possible for scientists to study stars and establish whether they are orbiting or flying around a given black hole. It is also important to note that when a star and a black hole come close together, what is emitted is a high-energy light. Although it is not possible to see this light using human eye, it can be seen with the use of space telescopes and satellites.

Also of importance to know is whether a black hole is capable of destroying the earth. According to NASA, it is not possible to have the earth fall into a black hole since none of them (black holes) is as near the solar system as to be able to swallow the planet. The same applies even in a case where a black hole containing the mass of a sun takes its place. Since the gravity of the black hole would be equal to that of the sun, all planets including the earth would orbit it in the same manner they do to the earth presently. It is also noteworthy that it is not possible for the sun to become a black hole, since its smaller size would not allow it to do that.


Galaxies refer to sprawling space systems that are comprised of gas, dust, and many stars. It is not possible to count the number of galaxies, but it is thought that the observable universe could have as many as 100 billion galaxies on its own. Some galaxies resemble that of the Earth-the Milky Way-whereas there are others that show some difference. “Small galaxies” are those containing fewer than one billion stars. In Earth’s galaxy, the sun is only one out of approximately 100 billion stars. There are three main types of galaxies namely elliptical galaxies, spiral galaxies, and lastly, irregular galaxies. The Milky Way is a spiral galaxy and this type of galaxies are comprised of a flat disk with the center bulging out and surrounded by spiral arms. The disk is composed of planets; stars, gas and dust, and all of them regularly rotate around the center of the galaxy. The spinning motion, which is usually at hundreds of kilometers each second, often results in the disk’s matter adopting a spiral shape similar to a cosmic pinwheel.

Elliptical galaxies have an elliptical shape, in which they are roughly round, but extend longer on one axis compared to the other. They contain older stars that can be as many as one trillion, but they also have a little dust and other matter in between the stellar. The stars revolve around the galactic center in a manner similar to that of the spiral galaxies although they do it in more randomly. There are not many stars known to show the characteristics of elliptical galaxies.

The third category is that of galaxies that are neither elliptical nor spiral. These irregular galaxies do not have a distinct form and they appear misshapen, and this has been attributed to the fact that they are found within the gravitational influence of nearby galaxies. There also exist galactic mergers whereby some galaxies occur as a component of larger associations referred to as super clusters, groups or just clusters. It is thought that galaxies were formed immediately following a cosmic “big bang”, the same one that led to the formation of the universe billions of years ago.


As seen in this essay, quite a lot is known about stars, black holes, and galaxies. Scientists have gone to a considerable length is describing the formation, appearance, and even lifecycle of the three. The various types have also been described, as well as the forces behind their changing phases. This information has been instrumental towards understanding the universe and solar system. It is also worth noting that the three objects are interrelated. The stars lead to the formation of the black holes whereas galaxies host stars as one of their components. More studies and observations continue being made and it will be interesting to learn more about the three.


Related essays