A black hole was found near to the Earth: HR 6819 star system

                                 

                                   

Fig: Two stars orbiting a black hole

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Overview

Black hole is an area of space where gravity is so strong that even light cannot escape from it. It is formed when a star having mass equal to or more than 3.2 times of solar mass (mass of the sun) collapses at the end of its life cycle. The boundary of a black hole from which escape is not possible is called even horizon. Astronomers have predicted that there is a supermassive black hole in the center of each galaxy.

All of us have seen the first ever image of the black hole of Messier 87 galaxy in the history on 10 April 2019. But this is not an actual picture of black hole because it does not allow light to escape from it due to which we cannot capture its real image. This red or orange coloured ring indicates the extreme heat of dust particle and gas around it.

Fig: Shadow image of black hole of 

M-87 Galaxy

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Here, we are going to discuss about the HR 6819 system which contains a black hole near to the Earth.

Introduction to HR 6819

HR 6819 is a triple star system which is situated in the south western part of the Telescopium constellation. In triple star system, each of the star orbits a mass of its centre.

There is a stellar mass black hole in the centre which is orbited by a star and an outer star is also orbiting them as seen in picture below:

Fig: Black hole of HR 6819

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How far is this black hole from us?

HR 6819 system is about 1000 light years away from the Earth. It sounds this system is very close to us but light year is the distance measurement unit of astronomical object (i.e. the distance travelled by light in a year) which is equal to 9.46*10^12 K.M. So, 1000 light years is equal to 9.46*10^15  K.M. that means the black hole of HR 6819 star system is very far from us. 

How was this black hole detected?

Black holes can be detected from their influence on its surrounding. Astronomers have started the study of HR 6819 system many years ago. They used 2.2 meter telescope at La Silla Observatory in Chile to detect two stars.

Fig: La Silla Observatory in Chile

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But they were shocked to know that the inner star was orbiting a mass which was hiding in the system. Finally, they were concluded that this third object is a black hole formed from the collapse of a dying star. However, we cannot see the ring of this black hole because its mass is only four times of solar mass but the black hole of Messier 87 galaxy is so huge that its mass is about 6.6 billion times of solar mass. 

Can we see it with our naked eye?

Yes, we can see the stars of HR 6819 system with our naked eye but from only in the southern hemisphere in the night sky of the Earth. It is a surprise for us that HR 6819 is the first system with a black hole can be seen with our naked eye.

But we cannot see this black hole as light cannot escape from it, so that, we are able to see only its two stars.

Fig: HR 6819 star system in the sky

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Is the black hole of HR 6819 dangerous for our Earth?

In a single sentence, its answer is no. Firstly, this black hole is very far from our Earth. Secondly, it does not release bright x-rays while interacting with its orbiting stars.

Also, HR 6819 system is moving farther away at the rate of 9.4 kilometer per second. Since this black hole won’t harm us, so that, we have nothing to be worried for this black hole.

Conclusion

The black hole of HR 6819 is not only a black hole near to the Earth. There might be many black holes around our solar system which have not been detected yet. Astronomers have estimated that there are ten million to a billion of black holes in our Milky Way galaxy.