What is Astrophysics?

Astrophysics is defined as the branch of astronomy that employs the principles of physics and chemistry to ascertain the nature of astronomical objects, rather than their positions or motions in space.

It applies chemical and physical laws to explain the birth, life, and death of celestial objects like stars, planets, galaxies, nebulae, black holes, quasars, and many others in the universe.

 

(Image: Hubble Telescope)

History of Astrophysics

It all began with Sir Isaac Newton. While astronomy is one of the oldest sciences, the theoretical concept of astrophysics was given by Newton. Prior to Newton, many astronomers described the motion of heavenly bodies using complex mathematical models but without any physical basis.

Newton proved that celestial objects and earth and its objects are subject to the same physical laws. He did this by showing that the motion of the moon and planets, and the trajectory of a cannonball around the Earth can be easily explained simultaneously by a single well-defined theory.

The one fact that most completely separated Newton's model from the previous ones is that it is predictive as well as descriptive. Based on aberrations on the orbits of Uranus, astronomers predicted the position of a new planet in our solar system, which was then observed and named Neptune later.

Being predictive and descriptive is the sign of mature science, and astrophysics is in this category. 

Some Revolutionary Milestones In Astrophysics

Dark Lines In Solar Spectrum

In 1802, Sir William Wollaston and Joseph Fraunhofer were performing experiments on the splitting of white light coming from the Sun and they discovered some dark lines (known as Fraunhofer lines) in the spectrum as shown below. 

(The Fraunhofer Lines)

When Sunlight passes through a prism, it splits into colors of the rainbow in the visible range wavelength. These colors are known as the spectrum of the Sun. Initially, physicists thought that this spectrum is continuous which means there is no sharp edge or a gap between different colors of the spectrum. But when closely observed, there were many dark lines.

Later on, scientists found that these are absorption lines caused by impurities such as calcium, magnesium, iron, sodium, etc. The chief element in the Sun is hydrogen and helium,  and the impurities in minuscule quantities absorb the light coming from inside at specific wavelengths, resulting in dark lines.

Doctoral Thesis of Cecilia Payne

In the early 1920s, Cecilia Payne, in his doctoral thesis, discovered by using spectroscopy that, stars predominantly consist of Hydrogen and helium. It was one of the remarkable thesis in astrophysics and was the great landmark in the development of astrophysics as it laid the foundation for stellar evolution.

Source of Energy Of The Star

Now it was Einstein's era when his world-famous energy-mass equivalence equation gave astrophysicists the first clue to what the true source of energy might be in stars. As nuclear physics, quantum physics and particle physics grew in the first half of the 20th century, it became possible to formulate theories for how nuclear fusion can be the source of energy in stars. These theories describe how stars form, live, and die and also successfully explain the observed distribution of types of stars, their spectra, luminosity, ages, and other feature.

Hubble's Law And The Expanding Universe

For a long time, humans believed that the universe contains only one galaxy i.e. our Milky Way. Although we had images of other galaxies such as the Andromeda galaxy, the Magellanic Clouds, etc... they were believed to be stellar systems inside our Milky Way itself. But soon, astronomers realized that there are many more galaxies in the universe. Astrophysics took yet another turn with the work of Edwin Hubble. Hubble postulated a law that states that the farther a galaxy is deep in space, the faster it moves away from us. This proved that the universe is expanding.

Detection of Gravitational Waves

Gravitational waves are produced when two compact objects like black holes collide with each other. These waves are quite difficult for us to detect because, the objects which produce them are far away from our reach, and by the time these waves travel to the Earth, become very weak to be detected.

Illustration of gravitational waves (Image: LIGO/T. Pyle)

Gravitational waves provide complementary information to that provided by other means. By combining observations of a single event made using different means, it is possible to gain more complete knowledge of the source's properties (This is know as multi-messenger astronomy).

Detecting and analyzing the information carried by gravitational waves is allowing us to observe the universe in a way never before possible, providing astronomers and other scientists with their first glimpses of literally sightless wonders.

The First Black Hole Image

Two years ago, the Event Horizon Telescope published the first image of the black hole at the heart of the Messier 87 (M87) galaxy. Besides the detection of gravitational waves, it was the most notable event in astronomy so far this century. Currently, more telescopes are being investigated around the world to improve image quality and photograph more black hole candidates.

The First Black Hole Image (EHT)

Are Astronomy, Cosmology, and Astrophysics Same?

Now we know what astrophysics is. Now let's learn how it differs from his brothers' astronomy and cosmology. 

 Astronomy is the branch of science that deals with the motion and relative positions of the celestial bodies. This includes predicting the positions of planets, eclipses, meteor showers, etc. Astronomy focuses primarily on celestial mechanics and optics to learn the positions and composition of some celestial objects. 

Cosmology is the study of the origin, evolution, and ultimate fate of the universe. It studies the universe on a larger scale. It studies the universe as a whole. Cosmology differs from astronomy in that the former is concerned with the Universe as a whole while the latter deals with individual celestial objects. 

• Astronomy measures positions, luminosities, motions, and other characteristics 
• Astrophysics creates physical theories of small to medium size structures in the universe 
• Cosmology does this for the largest structures and the universe as a whole. 

 In the rigid sense: 

 In practice, the three professions form a close-knit family. Ask about the location of a nebula or the type of light it emits, and the astronomer can answer first. Ask what the nebula is made of and how it was formed, and the astrophysicist will get in touch. Ask how the dates fit with the formation of the universe and the cosmologist is likely to intervene. But be careful: two or three of these questions can enter the conversation at the same time! 

 Astrophysics is the physics of stars and other distant bodies in the universe, but it also applies close to home. According to the Big Bang theory, the first stars were made up almost entirely of hydrogen. The nuclear fusion process that drives them breaks down hydrogen atoms to form the heavier element helium. In 1957,  Geoffrey and Margaret Burbidge's team of astronomers, along with physicists William Alfred Fowler and Fred Hoyle, showed how, with increasing age, they produce heavier elements, which they pass on in increasing amounts to later generations. of stars. Earth's elements such as iron (32.1 percent), oxygen (30.1 percent), and silicon (15.1 percent) are only created in the later stages of the life of newer stars. . Another of these elements is carbon, which together with oxygen makes up most of the mass of all living things, including us. Astrophysics tells us that we may not all be stars, but we are all stardust.

Goals Of Astrophysics

Astrophysicists seek to understand the universe and our place in it. At NASA, the goals of astrophysics are "to discover how the universe works, explore how it began and evolved, and search for life on planets around other stars," according to NASA's website.