Space

Why are Stars So Bright?

January 5, 2021January 5, 2021 by Manisha Mani

Walk outside on a dark night and look up into the night sky. On seeing a clear night sky, you will notice beautiful stars shining in the night sky. A glance at the night sky shows that some stars are much brighter than others. However, the brightness of a star depends on its composition and how far it is from the planet.

All stars, and our own Sun is a perfect example of hot balls of glowing plasma that held together by their own gravity. And the gravity of a star is huge and potent. Stars with a progressive nature usually are crushing themselves inward. The gravitational friction of this affects their interiors to boil up. A star like the Sun is nearly 5,800 Kelvin at its surface. However, at its core, it can be around 15 million Kelvin.

The extreme pressure and temperature at the core of a star enables nuclear fusion reactions to occur. This is where atoms of hydrogen are combined into atoms of helium with various different stages. This reaction emits a huge quantity of energy in the form of gamma rays. These gamma rays are trapped inside the star, and they push in the outer direction against the gravitational contraction of the star. And that explains the reason why stars hold to a certain size, and not continue shrinking. The gamma rays jump around in the star, in the attempt to get out. They are soaked up by one atom, and then it releases again. This can happen more than twice within a second. A single photon can take around 100,000 years to get from the core of the star to its surface.

When the photons come to the surface, they have strayed some of their energy. They further turns into becoming visible light photons, and not the gamma rays as they started out. These photons jump down the surface of the Sun and move out in a straight line into space. Also, they can move for ever if they don’t run into anything.

Suppose you are looking at Sirius, which is located around 8 light-years away. Now, while doing that, you are looking at the photons that left the surface of the star 8 years ago. And further it kept moving through space, without running into anything. Your eyeballs are the first thing those photons have come across. So what exactly makes the stars look brighter?

Brightness of the Star

Astronomers are keen while distinguishing between the luminosity of the star and the amount of energy. Here, the energy is what happens to reach our eyes or a telescope on Earth. Stars are representative in nature. It is democratic in how they form radiation. They have a tendency to emit the same proportion of energy in every direction in space. Hence, only a small fraction of the energy given off by a star basically reaches an observer on Earth. So the amount of a star’s energy that reaches a given area in each second is called its apparent brightness. If you look at the night sky, you see a broad range of apparent brightnesses among the stars. Most stars, in fact, are so faded that you need a telescope to spot them.

The stars with the same luminosity would have been like standard bulbs with the same light output. We could use the difference in their apparent brightnesses to tell how far they are. Now, assume your friend to be in a big concert hall which is dark except for a few dozen 25-watt bulbs placed in fixtures around the walls. Since they are all 25-watt bulbs, their output of energy would seem the same. However, they would not seem to have the same apparent brightness from the corner where your friend is standing. Those close to your friend would appear brighter whereas those who are standing far would appear. In this way, your friend can tell which bulbs are closest to you. Similarly, if all the stars had the exact same luminosity, we could have straight away concluded. As the brightest-appearing stars were close by and the dimmest-appearing ones were far flung away.

To understand it better, the energy we receive is inversely proportional to the square of the distance. Let’s say we have two stars of the same luminosity and one is twice as far away as the other. In that case, it will look four times dimmer than the closer one. If, for example, it is three times distant, it will look nine times dimmer, and so on.

It’s sad that not all the stars have the same luminosity. Therefore, this means that if a star looks faded in the sky. Now just due to its low luminosity, we cannot tell whether it appears faded or not. However, it is relatively close. And in order to examine the luminosities of stars, we must first recompense for the dimming effects of distance on light. And to do that, we must know how far away they really are. Distance is considered to be the most difficult of all astronomical measurements. Now, let’s detail out how astronomers specify the apparent brightness of stars.

The Magnitude Scale

The process of measuring the apparent brightness of stars is termed as photometry, meaning “to measure”.

The brightest stars, those that were traditionally known as first-magnitude stars, did not turn out to be as same in context to brightness. For instance, the brightest star in the sky, Sirius, sends us up to 10 times as much light as the average first-magnitude star. On the modern magnitude scale, it has been assigned a magnitude of −1.5. Other objects in the sky can appear even more shining. Venus at its brightest carried a magnitude of −4.4. The Sun, on the other hand, carried a magnitude of about −26.8. The most essential fact to remember when using magnitude is that the system runs in a reverse direction. The larger the magnitude, the fainter the object we are looking at.

The magnitude scale is helpful for discovering visual astronomy. However, it is not used at all in newer branches of the field. In case of radio astronomy, no similar magnitude system has been described. Instead, radio astronomers measure the amount of energy being collected each second with the help of each square meter of a radio telescope. They further details the brightness of each source in terms of, let’s say, watts per square meter.

With a similar situation, most researchers in the domain of infrared, X-ray, and gamma-ray, astronomy use energy per area per second instead of taking magnitudes to tell the results of their measurements. Therefore, the luminosity is an essential feature that tells us a lot about the object in question. While the energy that reaches Earth is an accident of cosmic geography.

Possibly the simplest measurement of a star is its apparent brightness. Here, apparent brightness suggests how bright the star seems to a detector here on Earth. The luminosity of a star, therefore, is the amount of light it emits from its surface. The key distinction between luminosity and apparent brightness depends on distance. Another way to look at these measurements is that the luminosity is an inherent characteristic of the star, which tends to explain that everyone who has some means of quantifying the luminosity of a star should find an identical value. However, apparent brightness is not an inherent characteristic of the star. It relies on the location at which you have been placed. Thus, everyone will measure a different apparent brightness for the same star if they all are located at different distances away from that star.

For the likeness with which you are close, let’s take the headlights of a car as an example. Suppose the car is located at some far flung distance. And even if its high beams are on, the lights will not appear too bright. The car, for example, drives and crosses you within 10 feet. In doing so, its lights may appear so bright that the light will start to blind your vision at that point. Now let us put it in another way. Given two light sources with the same luminosity, the closer light source will be even brighter in appearance. However, not all light bulbs comprises of the same luminosity. If, for example, you put an automobile headlight 10 feet away and a flashlight 10 feet away, the flashlight will appear fainter because its luminosity is relatively smaller.

Conclusion

Stars, thus, have a wide range of apparent brightness measured on our very Earth. The variation in their brightness seems to vary by both differentiations in their luminosity and distinctions in their distance. An inherently dim, nearby star can appear to be just as bright to us on Earth as an inherently luminous, distant star. Therefore, there is a clear mathematical relationship which matches these three quantities, that is, apparent brightness, luminosity, and distance for all light sources, which also comprises of stars.