If you're curious about how scientists understand stars, it's all about luminosity. This term refers to how much electromagnetic radiation a star gives off in a certain amount of time. But here's the thing – when we hear "luminosity", we usually think of visible light. In reality, it includes all types of electromagnetic radiation. To really understand a star, we need to measure all the different frequencies of radiation it emits, not just the ones we can see.

Luminosity and distance

It's important to keep in mind that our measurements of luminosity are affected by our position in the universe. This is because stars and other astronomical objects emit radiation in all directions, and it spreads out rapidly. As a result, we can only observe a small part of the radiation they emit, and we have to use that information to make educated guesses about the rest.

Luminosity and extinction

Space might seem empty, but it's actually full of things like space dust that can get in the way of detecting radiation from distant objects. This can cause "extinction", which means that the intensity of the radiation decreases. Extinction affects high-energy radiation (like gamma and x-rays) more than low-energy radiation (like radio waves and infrared radiation).

Because of extinction and our distance from the object we're observing, our measurements can be influenced by our position in the universe. But by using direct and indirect methods, we can still estimate these quantities accurately. Even though luminosity is somewhat subjective, it's still a useful measure of the intensity of radiation we detect from Earth.

Classifications by luminosity

One classification system for stars and other astronomical bodies is based on their luminosity, or the intensity of electromagnetic radiation they emit. Another related system is the spectral classification of stars.

The luminosity classification system categorizes stars into different classes based on how bright they are. The classes are denoted by letters, with the brightest stars being classified as "I", and the faintest as "V". This system is known as the Yerkes luminosity classification system, and it is based on the work of astronomer William W. Morgan and his colleagues at the Yerkes Observatory in the early 20th century.

The spectral classification of stars, on the other hand, is based on the types of spectral lines present in their spectra. The classes are denoted by letters from O to M, with O being the hottest and M being the coolest. This system is also known as the Harvard spectral classification system, and it was developed by astronomers Annie Jump Cannon and Edward C. Pickering in the early 1900s.

The luminosity and spectral classification systems are related because the luminosity of a star is related to its temperature, which in turn is related to the types of spectral lines present in its spectrum. By studying these classifications, astronomers can better understand the properties and behavior of stars and other astronomical objects.

The luminosity classification of Hipparchus

Hipparchus, a Greek astronomer from the 1st century BCE, was one of the first to attempt to classify stars by their brightness. He created a system of numbers ranging from one to six, which he called "magnitude". In this system, the brightest stars were assigned a magnitude of 1, while the faintest stars that he could barely see were given a magnitude of 6.

This was an important step towards understanding the nature of stars, and it laid the foundation for future classification systems. However, this early system was limited in that it only took into account the brightness of stars as they appeared to the naked eye, and did not consider other factors such as distance or intrinsic brightness.

Over time, astronomers developed more sophisticated classification systems that took these factors into account. The luminosity classification system and spectral classification system are two examples of such systems that have been developed and refined over the years.

The actual concept of luminosity

You're absolutely right! The luminosity of stars and other astronomical objects is a concept is difficult One calculate luminosity is based on the concept of black body radiation, which assumes that the object is a perfect emitter of radiation. This model takes into account the object's surface area and temperature to calculate its luminosity.

However, measuring luminosity accurately is a challenging task, as it requires measuring the object's emission in all directions. Additionally, the perceived luminosity of an object also depends on its distance from the observer. As a result, astronomers often use a combination of different methods to estimate the luminosity of stars, such as measuring their brightness in different wavelengths of light or studying their spectral lines. These methods can help astronomers better understand the properties and behavior of stars and other astronomical objects.

Yerkes classification

The following table has the Yerkes luminosity classification chart, which assigns each star a number based on their luminosity. It is the most widely used classification by luminosity for stars.

Magnitudes and luminosity

You are correct! To simplify the measurement and comparison of the brightness of astronomical objects, astronomers use the concepts of absolute and apparent magnitude. These scales allow us to compare the brightness of objects by using a common reference point.

The absolute magnitude is the brightness an object would have if it were located at a distance of 10 parsecs from the observer. This provides a standardized measure of the object's intrinsic brightness. On the other hand, the apparent magnitude is the brightness of the object as it appears from Earth or any other location.

However, as, the and of light by dust and other materials in space is still present To account for this, astronomers use models to estimate the extinction and correct for its effects.

Additionally, telescopes are limited in their ability to reach and measure astronomical objects at a distance of 10 parsecs. Therefore, the estimation of absolute and apparent magnitude requires complex models that take into account various factors, such as the object's distance, luminosity, and spectral properties. These models allow astronomers to accurately estimate the brightness of astronomical objects and gain insight into their properties and behavior.

The relationship between luminosity and spectral classifications

If we fix area A in the luminosity formula, we just have a dependence on the temperature of the star or astronomical object. Hence, under the assumption that stars emit as black bodies, using quantities such as the absolute magnitude allows us to consider only their temperature.

In addition, there is a phenomenon called Wien’s law, which takes into consideration how a black body emits radiation at a certain temperature. It turns out that the intensity of emission of frequencies depends on the temperature.

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Since stars emit in all frequencies (with different intensities depending on their temperature), we observe a dependence of their colour on their temperature. This gives rise to the concept of stellar spectral classification.

Although we cannot here explore how stellar spectral classifications work, it is important to be aware of the system and that temperature is correlated with luminosity, as illustrated by diagrams such as the Hertzsprung-Russell diagram (see also the table below). The luminosity of a star varies throughout its life and depends on certain phases like pulsating phases, the giant phase, or the white dwarf phase.

Computations with classifications by luminosity

Sirius is the brightest star that can be seen from the earth. It has a luminosity on its surface of 25.4 times the luminosity of the sun (L0), a radius of 1.711 times the radius of the sun (r0), and is 8.61 light-years away from the earth. Sirius is a main-sequence star (V).

We can compute the temperature for both stars, assuming they behave like black bodies. Indeed, knowing that the luminosity of the sun has an approximate value of 3.83·1026 W and the radius of the sun has an approximate value of 6.96·108 m, we can use the following formula, which comes from the formula of luminosity, using the fact that stars are spherical bodies, so we can easily compute their surface area:

This yields a temperature of 9904 K for Sirius and 3673 K for Antares. These values are very close to the measured ones, which are 9940 K and 3660 K, respectively. Since Sirius has a higher temperature than Antares, its colour is much bluer.

We finally turn to compute the luminosity of these two stars as perceived on the earth. The formula is:

Here, r is the radius of the star, while d is the distance of observation, which, in this case, is the distance to the earth. L is the luminosity of the star, and Lp is the luminosity observed from a distance d, which is lower due to the spherical spreading of electromagnetic radiation.

Using the values, we get observed luminosities per area of 1.14·10-7 W/m2 and 8.54·10-8 W/m2, respectively, which explains why Sirius is brighter than Antares as seen from the earth. This is related to the concept of magnitude.

Key takeaways Luminosity is the total amount of electromagnetic energy emitted by a body per unit of time. Measurements of luminosity in space are difficult due to the spreading of energy and extinction. There are logarithmic scales related to luminosity that also take into account the spatial spread of the radiation. These are known as magnitude scales. Thanks to the assumption of black body radiation, we can deduce many properties of stars by knowing their magnitude, distance, etc.

Classification by Luminosity

How do you classify a star’s luminosity?

The most widely used system nowadays is the Yerkes classification, which assigns a certain number and letter to a star depending on its luminosity.

Are stars classified by luminosity?

Yes, they are, since it gives information about the electromagnetic radiation they emit, their temperature, their size, etc.

What classification of stars has the lowest luminosity?

In the Yerkes classification, the lowest luminosity is associated with white dwarves (class VII).

What are the 7 spectral classes of stars?

According to the stellar spectral classification, they are: O, B, A, F, G, M, and K.

What are the three main luminosity classes?

The three main luminosity classes are giants (class III), main-sequence stars (class V), and white dwarves (class VII).

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