What is Spectroscopy?

The core of a star generates a continuous light spectrum due to blackbody radiation, as shown in the diagram below. As the light passes through the stars atmosphere, the electrons in the atoms present absorb certain wavelengths of light, leaving lines missing on the spectrum. If the light passes through a cloud, the atoms in this cloud act in the same way. The lines absorbed will change depending on the elements present in the star’s atmosphere which allows these spectrums to be used as a ‘fingerprint’ to identify what atoms are present in different stars. This is Spectroscopy – the measurement of spectra produced when matter interacts/emits electromagnetic radiation.

Spectral Classification – Hertzsprung-Russell Diagram – NAAP

Black holes

A black holes is formed when the center of a massive star collapses in on itself. Since the matter is very densely packed, the gravitational pull inside the black hole is so strong that nothing can escape, not even light! Three black holes have been found by Gaia so far. It’s very hard to detect black holes.

The event horizon is the point that marks the outer edge of the black hole and after this point there is no returning. General relativity can be used to describe the formation of black holes. When the star collapses, it becomes smaller and more dense. The closed trapped surface points all the paths towards the center. At the central point, collapsing matter is crushed to infinity. Black holes are theoretically predicted to eventually evaporate over a long time. When a particle and it’s antiparticle momentarily emerge in and out of existence near the event horizon, one can get pulled back in by gravity, while the other goes off into space. Overtime, these escaping particles cause the black hole to slowly evaporate.

New picture of famous black hole reveals its swirling magnetic field | New Scientist

If light can’t escape, how are these two factors related?

The problem is that black holes aren’t giving off light, so how can spectroscopy be used to understand them? The answer is in the material surrounding the black holes. An accretion disk can form around black holes, it is a somewhat flat sheet of gas and dust. The interstellar medium can therefore be studied with spectroscopy and it is possible to discover what type of materials fill the space around black holes. This gives us a better understanding about black holes and gives more information about them to aid future research.

From Fabian (2000), X-ray spectroscopy detects broad iron Kα emission lines from material near black holes. The lines are broadened and skewed due to doppler shifting and gravitational redshifts. This allows the effects of general relativity to be further researched near the event horizon. Geometry and motion of the accretion disk can be determined through the shape of these iron lines. If the iron lines are extremely redshifted, this could suggest a spinning black hole.

In conclusion, spectroscopy is a great tool in discovering more about black holes and using it alongside other methods will lead to a deeper understanding of them. It is an ongoing research and a very exiting time to be studying Physics!