When you look at a star, there are a few features that you may notice. One obvious thing that you may notice is that some stars are brighter than others. But why is this the case?
In this article, we’ll be taking a closer look at why some stars are brighter and what affects the brightness we see from Earth.
Two main factors affect the brightness of the stars. These factors are luminosity and distance from Earth. Some stars are intrinsically brighter, while others are relatively dim in comparison.
Proximity is also a huge factor. For example, the sun is the nearest star to Earth and appears brightest. However, it is not the brightest star. It has a medium brightness when compared with others.
In this article, we’ll be taking a closer look at how we measure each star’s brightness and magnitude. We’ll also examine variable stars and how we have measure star brightness.
Measuring the Brightness of Stars
Hipparchus was the first person in history to rank stars in order of brightness in the second century BC. However, this was only a catalog. It was Ptolemy who developed our current magnitude system.
There are six magnitudes, according to Ptolemy. The first magnitude contains the brightest stars, while the sixth was the dimmest.
The system was refined as time passed, even more so in the 16th century when the first telescopes were made.
Once telescopes were invented, we could identify stars dimmer than the sixth magnitude, creating new tiers. Soon, a rule was developed to show a five-magnitude difference in brightness on a 100 to 1 scale.
Over the years, the magnitude was developed to measure other celestial objects, such as the International Space Station, the sun, and the moon. Because some objects are so bright they are attributed negative values. For example, the apparent brightness of the Sun is -26.8 while the Moon is -12.7 at full Moon.
What Is The Apparent Magnitude?
Measuring the true brightness of a star is difficult. This is because we only have the option to view them from Earth. To determine true magnitude (or brightness) of a star as we just consider distance. For this reason, it is convenient to use apparent magnitude.
The apparent magnitude of a star is how bright it appears when observed from Earth. If a star of a given brightness is placed close to Earth it will be brighter than if placed a long distance from us.
Some stellar objects will appear brighter because they’re closer to us. However, some brighter stellar objects will appear dimmer because of the further distance away.
The Inverse Square Law describes why stars get dimmer the further away they are observed. In simple terms, if you double the distance between you are the star the same amount of light is spread across four times the area. By doubling the distance a star will have a quarter of the brightness.
Amateur astronomers generally use apparent magnitude as the calculations to determine absolute brightness are too complicated.
Professional astronomers can determine the absolute magnitude of stars due to the equipment they have at their disposal. By using the correct formulas and calculating the star’s distance, they can discover the absolute magnitude of a star.
What Is The Absolute Magnitude?
Absolute magnitude measures how bright a star is at a standard distance.
In most instances, a star’s absolute brightness results from its mass. The heavier a star, the more hydrogen it has to fuse to helium and other elements. Having a higher mass also means that the temperature at the star’s core is hotter and at a higher pressure. With more energy being produced, the star’s surface temperature will be higher. A higher surface temperature will emit more light.
As well as emitting more light, large stars emit more blue light, so they appear blue. The color of the light emitted is related to the star’s surface temperature.
We mentioned early that the star’s brightness diminishes with distance. Hence, to calculate absolute brightness, we must define a distance at which we base our calculations. This distance is set at 10 Parsecs, or 32.6 light years.
Stars emit light at many wavelengths. Also, different sized stars are brighter at different wavelengths. As such we must consider the wavelength at which we measure absolute brightness. For the calculations, astronomers use light in the ‘visual’ range. This means we use the same light we see with our eyes.
To calculate the absolute brightness of a star, we must also know the distance to it. Astronomers have several tools they can use to determine distance. These techniques use things like parallax and variable stars. To discover more about how distances to stars are determined you can read How Far Away Are Stars?
With the apparent brightness and distance to a star known, it is a simple calculation to determine the star’s absolute brightness.
As a side note, every star we see in the night sky is in our galaxy.
What Are Variable Stars?
Finally, we need to consider stars, known as variables. These stars have brightnesses that change over time. These aren’t rare, as over 100,000 stars have been classed as a variable.
Even our sun is seen as a variable, with a changing magnitude of 0.1% every 11 years. This is a result of sunspot activity.
The speed of each star’s brightness will vary depending on several factors. There are long-term and short-term variables. Most stars will vary in magnitude over the long term. Astronomers are much more interested to study short-term variables.
There are two types of short-term variables when you look at these stars. The first form is an intrinsic variable, meaning the stars will vary depending on the light they produce. For instance, a star may shrink or swell.
The second form is extrinsic. These stars change brightness due to something stopping the light from reaching us. This might be due to things like sun spots or planets passing in front of it. The light produced by the star does not change.
An interesting feature of variable stars is that certain types can be used to measure distances. This is because the rate they vary is related to their mass. With knowledge of their mass, their absolute magnitude can be determined. Knowing their absolute magnitude, you can use their apparent luminosity to calculate distance. For more information on this and other techniques used to measure distances see How Far Away Are Stars?
Final Thoughts
As you can see, several factors will affect the brightness level of each star. You need to consider the distance and the luminosity of each one.
Due to this, it can be complex for amateur astronomers to identify the absolute magnitude of a single star. So, most will use apparent magnitude for their observations.
If you want to know more about stars, telescopes, and everything in between, please check out the rest of our site.
We have plenty of articles to help you understand astronomy and clear up any other questions you may have about your telescope and how it works.
The absolute brightness of a star is directly related to its surface temperature. Generally more massive stars have higher temperatures and are brighter as a result.
The apparent brightness of a star depends on its luminosity and the distance it is from Earth.
However, it can be difficult to analyze the absolute magnitude due to the difficulty of measuring the distance to stars.
Are Some Stars Brighter Than The Sun?
Every star in the sky is bigger and brighter than the sun.
There are 5,000 stars or more that are brighter than magnitude 6 and only a handful of faint stars that are roughly the same size and brightness as our sun.
Most of them are bigger and brighter.
Are Older Stars Brighter?
Stars change brightness and color as they age, making it simpler for scientists to determine their age. As a star grows older, it will brighten and become redder.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
