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Astronomical Objects

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Astronomical Objects

An astronomical object is a certain astronomical structure undergoing one or several processes that can be studied in a simple way. These are structures that are not big enough to have more basic objects as their constituents and not small enough to be part of another object. This definition relies crucially on the concept of simple, which we are going to illustrate with examples.

Consider a galaxy such as the Milky Way. A galaxy is a gathering of many stars and other bodies around a nucleus, which, in old galaxies, is usually a black hole. The basic constituents of a galaxy are the stars, no matter their stage of life. Galaxies are astronomical objects.

However, an arm of a galaxy or the galaxy itself is not an astronomical object. Its rich structure does not allow us to study it with simple laws that do not rely on statistics. Similarly, it does not make sense to study relevant astronomical phenomena by just looking at the layers of a star. They are entities that do not capture the full complexity of the processes happening in a star unless considered together.

Thus, we see that a star is a perfect example of an astronomical object. Simple laws capture its nature. Given that at astronomical scales the only relevant force is gravity, this concept of an astronomical object is strongly determined by the structures formed by gravitational attraction.

Astronomical Objects. Veil nebula. StudySmarterFigure 1. Veil Nebula (a structure that is too complex to be considered an astronomical object). Source: NASA Hubble Space Telescope, Flickr (CC BY 2.0).

Here, we only deal with old astronomical objects in that we only consider astronomical objects that have already undergone previous processes before acquiring their actual nature.

For instance, space dust is one of the most common astronomical objects, which gives rise to stars or planets over time. However, we are more interested in objects like the stars themselves rather than their early stages in the form of space dust.

What are the main astronomical objects?

We are going to make a list of astronomical objects, which includes some objects whose characteristics we wont explore before we then focus on three main types of astronomical objects: supernovae, neutron stars, and black holes.

Various astronomical objects

Beginning with various astronomical objects whose characteristics we wont explore, we find good examples in the astronomical objects closest to the earth, i.e., satellites and planets. As is often the case in classification systems, the differences between categories can sometimes be arbitrary as, for instance, in the case of Pluto, which was recently classified as a dwarf planet rather than a regular planet but not as a satellite.

Astronomical Objects. Pluto. StudySmarterFigure 2. Pluto. Source: Chris Meller, Flickr (CC BY 2.0).

Some other types of astronomical objects are stars, white dwarfs, space dust, meteors, comets, pulsars, quasars, etc. Although white dwarfs are the late stages in the life of most stars, their differences regarding their structure and the processes happening inside them lead us to classify them as different astronomical objects.

The detection, classification, and measurement of properties of these objects are one of the main goals of astrophysics. Quantities, such as the luminosity of astronomical objects, their size, temperature, etc., are the basic attributes we consider when we classify them.

Supernovae

To understand supernovae and the other two types of astronomical objects discussed below, we must briefly consider a stars stages of life.

A star is a body whose fuel is its mass because nuclear reactions inside it convert mass into energy. After certain processes, stars undergo transformations that are mainly determined by their mass.

If the mass is below eight solar masses, the star will become a white dwarf. If the mass is between eight and twenty-five solar masses, the star will become a neutron star. If the mass is more than twenty-five solar masses, it will become a black hole. In the cases of black holes and neutron stars, the stars usually explode, leaving behind remnant objects. The explosion itself is called a supernova.

Astronomical Objects. The life cycle of a star. StudySmarterFigure 3. The life cycle of a star. Source: NASA Goddard Space Flight Center, Flickr (CC BY 2.0).

Supernovae are very luminous astronomical phenomena that are classified as objects because their properties are accurately described by luminosity laws and chemical descriptions. As they are explosions, their duration is short in the time scales of the universe. It also does not make sense to study their size since they are expanding due to their explosive nature.

The supernovae that originated in the collapse of the core of stars are classified as types Ib, Ic, and II. Their properties in time are known and used to measure different quantities, such as their distance to the earth.

There is a special type of supernova, type Ia, that is sourced by white dwarfs. This is possible because, although low-mass stars end up as white dwarfs, there are processes, such as having a nearby star or system releasing mass, that can result in a white dwarf gaining mass, which, in turn, can lead to a type Ia supernova.

Usually, many spectral analyses are carried out with supernovae to identify which elements and components are present in the explosion (and in which proportions). The aim of these analyses is to understand the age of the star, its type, etc. They also reveal that heavy elements in the universe are almost always created in supernova-related episodes.

Neutron stars

When a star with a mass between eight and twenty-five solar masses collapses, it becomes a neutron star. This object is the result of complex reactions happening inside a collapsing star whose external layers are expelled and recombine into neutrons. Since neutrons are fermions, they cannot be arbitrarily close together, which leads to the creation of a force called degeneration pressure, which is responsible for the existence of the neutron star.

Neutron stars are extremely dense objects whose diameter is around 20 km. This not only means that they have a high density but also causes a rapid spinning motion. Since supernovae are chaotic events, and the whole momentum needs to be conserved, the small remnant object left behind by them spins very fast, which makes it a source of emission of radio waves.

Due to their precision, these emission properties can be used as clocks and for measurements to find out astronomical distances or other relevant quantities. The exact properties of the substructure forming neutron stars are, however, unknown. Features, such as a high magnetic field, the production of neutrinos, high pressure and temperature, have led us to consider chromodynamics or superconductivity as necessary elements to describe their existence.

Black holes

Black holes are one of the most famous objects found in the universe. They are the remnants of a supernova when the mass of the original star exceeded an approximate value of twenty-five solar masses. The huge mass implies that the collapse of the core of the star cannot be stopped by any kind of force that gives rise to objects like white dwarfs or neutron stars. This collapse continues to exceed a threshold where the density is too high.

This huge density leads to the astronomical object generating a gravitational attraction so intense that not even light can escape it. In these objects, the density is infinite and concentrated in a small point. Traditional physics is unable to describe it, even general relativity, which calls for the introduction of quantum physics, yielding a puzzle that is not yet solved.

The fact that not even light can escape beyond the horizon event, the threshold distance determining whether something can escape from the influence of the black hole, prevents useful measurements. We cannot extract information from inside a black hole.

This means that we must make indirect observations to determine their presence. For instance, active nuclei of galaxies are believed to be supermassive black holes with mass spinning around them. This comes from the fact that a huge amount of mass is predicted to be in a very small region. Even though we cannot measure the size (no light or information is reaching us), we can estimate it from the behaviour of the surrounding matter and the amount of mass causing it to spin.

Regarding the size of black holes, there is a simple formula that allows us to calculate the radius of the horizon event:

Here, G is the universal constant of gravitation (with an approximate value of 6.67⋅10-11 m3/s2⋅kg), M is the mass of the black hole, and c is the speed of light.

Astronomical Objects. Black hole. StudySmarterFigure 4. The most recent picture of what is believed to be a black hole. Source: NASA Goddard Space Flight Center, Flickr (CC BY 2.0).

Astronomical Objects - Key takeaways

  • An astronomical object is a structure of the universe described by simple laws. Stars, planets, black holes, white dwarfs, comets, etc., are examples of astronomical objects.
  • Supernovae are explosions that usually mark the end of the life of a star. They have well-known properties that depend on the remnant that they leave behind.
  • Neutron stars are a possible remnant of a supernova. They are, essentially, very small, dense, and fast-spinning bodies believed to be formed by neutrons. Their fundamental properties are unknown.
  • Black holes are the extreme case of a remnant of a supernova. They are the densest objects in the universe and are very mysterious because they don’t let any light escape. Their fundamental properties are unknown and have not been accurately described by any available theoretical model.

Frequently Asked Questions about Astronomical Objects

There are many: stars, planets, space dust, comets, meteors, black holes, quasars, pulsars, neutron stars, white dwarfs, satellites, etc.

There are techniques based on direct observation (with a telescope and knowing the distance between us and the object) or on indirect observation and estimation (using models for luminosity, for instance).

Yes, they are the basic constituents of galaxies.

By observation of the universe with telescopes in any frequency available and direct or indirect observation.

Yes, the earth is a planet.

Final Astronomical Objects Quiz

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A star is an astronomical object.

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Supernovae create most of the heavy elements of the universe.

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Neutron stars exist thanks to the degeneracy pressure.

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The spectral analysis of a supernova yields useful information.

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The radius of a black hole depends only on its mass.

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What is a black hole?

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It is an astronomical object generated after a supernova that is comprised of a point of infinite density that attracts everything, even light.

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What is a supernova?

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A supernova is an astronomical object caused by the collapse of the core of a star. Some are also generated by the explosion of a white dwarf.

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What is a neutron star?

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A neutron star is a very small and dense astronomical object left behind after a supernova that is mainly composed of neutrons.

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What is the current model for the active nuclei of galaxies?

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They are believed to be supermassive black holes with mass orbiting them.

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What is the name of the distance threshold beyond which not even light can escape the influence of a black hole?

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Horizon event.

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Does general relativity completely describe black holes?

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No, it fails to properly describe all their properties.

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Can we extract information from inside a black hole?

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No, not even light can leave its inside.

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Is the rotation of neutron stars precise or erratic?

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It is very precise, which is why they can be used as clocks.

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Do neutron stars have an intense magnetic field?

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Yes, they do.

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Does the radius of a black hole depend on its density?

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No, it only depends on the total mass of the black hole.

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