The Milky Way is one of the most fascinating and awe-inspiring sights in the night sky. As our home galaxy, it spans over 100,000 light-years and contains hundreds of billions of stars, as well as vast amounts of gas, dust, and other astronomical objects. From our perspective on Earth, the Milky Way appears as a band of hazy light that stretches across the sky, beckoning us to explore the mysteries of the universe. Join us on a journey to discover the wonders of the Milky Way and unlock the secrets of our cosmic home.

What is an astronomical object?

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.

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.

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 won’t explore before we then focus on three main types of astronomical objects: supernovae, neutron stars, and black holes.

However, we'll mention briefly some other astronomical objects whose characteristics we won’t explore in detail. 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, 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.

Figure 1. Pluto

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 star’s 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.

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:

\[R = 2 \cdot \frac{G \cdot M}{c^2}\]

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.