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How scientists determine the distance of cosmic objects and the age of images

Ever since the James Webb Space Telescope started taking pictures, one question on many people's minds is, how do scientists know that the stars and galaxies in the picture are 4.5 billion years old or 13.5 billion years old? Do they say these guesses? not at all.

Determining the distances of cosmic objects is one of the most important tasks in astronomy. Different types of procedures are followed to determine different distances.


When an object is viewed from a different angle, the position of the object appears to be different. Simply hold a pen or pencil in front of you to understand it. Then close the left eye and once the right eye and look at it. It will appear that the pencil is in two different positions. Our brain analyzes these two positions and we understand its actual position and distance. The greater the distance between these two observed points, the more precisely the position of the object can be determined. If a star is observed from any two places on Earth, this gap will be very small compared to the distance between the stars. So scientists use novel methods to increase this gap. Earth revolves around the sun and travels a huge distance in 6 months. So if a star is observed after 6 months then the distance between two observations will be very much and the position of the star can be determined accurately. After this observation is done then very simple mathematical calculation. A triangle is formed between the sun, the earth and the star. 

We know the distance of the earth's orbit and also get the value of the angle produced for the change of the stars. Now, using the formula tanθ=perpendicular/ground, we directly get the distance from the Sun to the observed star.

But the greater the distance, the smaller the angle. So this method cannot be used for distant stars or galaxies. So far its maximum limit is 10,000 light years. 

Standard Candle:

If two objects of the same brightness are placed one closer and the other farther away, the farther object will appear brighter. If the brightness of a near object is known, the distance can be determined by comparing the diminished brightness of the distant object. This method is used in astronomy. There are some objects that are used as standard candles.

Cepheid variable:

These stars regularly contract and expand. Brightness decreases the most when compressed and brightness increases the most when expanded. Brighter Cepheid variables have longer cycles and less bright ones have shorter cycles. From this feature, its actual brightness and distance can be determined. They can be easily distinguished from other stars because of their increased brightness. Its limit is 100 million light years. 

Type 1 A Supernova:

If a star in a binary star system becomes a white dwarf, its high gravitational pull causes material from the other star to be attached to it and the mass of the white dwarf increases. But according to the Chandrashekhar limit, the mass of a dwarf cannot exceed 1.4 solar masses. So when the mass becomes more than 1.4 solar masses it explodes. This is called a 'type 1A supernova'. Supernovae are much brighter, and can even become the brightest objects in the galaxy. So it can be detected from far away (even from about 10 billion light years away). All types of type 1 supernovae are about the same luminosity. So in this case also the distance of these cosmic objects can be determined by comparing the brightness. However, such supernovae are very rare. 

Red Shift:

This is the highest level of determining the distance of cosmic objects. Analyzing light produces different color wavelengths. Violet light has the shortest wavelength and red light has the longest wavelength. As an object moves away, the wavelengths of light emitted by that object become longer. So as the distance increases, the light from an object tends towards red. This phenomenon is called 'red shift'. The distance to a distant galaxy is determined based on how much redshift it undergoes. Once the distance of a star or galaxy is determined, the distance of objects around it can be determined with its help. Determine the distance to the next object again with the help of the new object. It can be compared to a ladder. With the help of one step, another step, another step with the help of a new step and so on.

CEERS 93316 is the most distant galaxy ever seen, was recently imaged by the James Webb Space Telescope. The Big Bang occurred 13.8 billion years ago and the image released is only 235 million years after the Big Bang.

So the galaxy should now be 13.565 light years from Earth right? But the interesting thing is that it is not correct.

The universe is constantly expanding. This expansion is not understood in the case of stars in the same galaxy or in nearby galaxies. Because this dilatation cannot separate nearby objects. But distant objects gradually move away from each other. The greater the distance from one galaxy to another, the faster they are receding.


Here, v is the velocity of the galaxy, d is the distance and H is the Hubble constant. That is, the velocity of the galaxies is proportional to the distance.  In the case of CEERS 93316, light started traveling from the galaxy, light started traveling from the galaxy 235 million years after the Big Bang, and that light has now reached us. After all these years, the galaxy is no longer the same. It is currently about 35 billion light-years away from Earth due to the expansion of the universe.

Similarly, the earlier image of the galaxy GLASS-z13 was 13.4 billion years old, but the galaxy is now about 33.3 billion light-years away from Earth.

Sources:  Wikipedia, Astronomydotcom, BBC

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