How can cool stars be luminous

The answer to this actually depends on a few factors. These include:. Our Sun's luminosity is about 3. The source of energy in stars is described on another page. What is the relationship between distance to a star and its brightness that is the intensity of light we receive from it? As we can see in the diagram below as we move away from a star the radiation it emits has to cover a greater surface area.

Imagine a sphere at a distance r from the star. Assuming a star is spherical and emits radiation equally in all directions the intensity of the energy at distance r over an area A will be I. If we double the distance from the star to 2 r this same amount of energy is now distributed over four times the surface area, that is 4 A.

This relationship for light or any electromagnetic radiation is called an inverse-square relationship. Astronomers can measure the distance to the nearest stars using a method known as trigonometric parallax. In fact Canopus is the second brightest star visible in the night sky after Sirius A whereas with an apparent magnitude of Example 2: Calculating brightness range for a variable star.

It was the first such star discovered and has given its name to a class of variable stars. The importance of these is discussed in a later section. In this type of problem we simply substitute in the two values for the apparent magnitudes for the same star so;. Example 3: Comparing two stars' luminosities.

How much more luminous is Betelgeuse than our Sun? Skip to main content. Australia Telescope National Facility. Accessibility menu. Interface Adjust the interface to make it easier to use for different conditions. Interface Size. High contrast mode This renders the document in high contrast mode. Invert colors This renders the document as white on black.

Disable interface animations This can help those with trouble processing rapid screen movements. The color of a star indicates its temperature. Blue-white stars are much hotter than the Sun, whereas red stars are cooler.

On average, the stars in this field are at a distance of about 25, light-years which means it takes light 25, years to traverse the distance from them to us and the width of the field is about Blue colors dominate the visible light output of very hot stars with much additional radiation in the ultraviolet.

On the other hand, cool stars emit most of their visible light energy at red wavelengths with more radiation coming off in the infrared Table 1. The color of a star therefore provides a measure of its intrinsic or true surface temperature apart from the effects of reddening by interstellar dust, which will be discussed in Between the Stars: Gas and Dust in Space.

Color does not depend on the distance to the object. This should be familiar to you from everyday experience. The color of a traffic signal, for example, appears the same no matter how far away it is. If we could somehow take a star, observe it, and then move it much farther away, its apparent brightness magnitude would change. To pin down this idea more precisely, recall from the Radiation and Spectra chapter that we know exactly how light fades with increasing distance.

The energy we receive is inversely proportional to the square of the distance. If, for example, we have two stars of the same luminosity and one is twice as far away as the other, it will look four times dimmer than the closer one.

If it is three times farther away, it will look nine three squared times dimmer, and so forth. Alas, the stars do not all have the same luminosity. Actually, we are pretty glad about that because having many different types of stars makes the universe a much more interesting place.

But this means that if a star looks dim in the sky, we cannot tell whether it appears dim because it has a low luminosity but is relatively nearby, or because it has a high luminosity but is very far away.

To measure the luminosities of stars, we must first compensate for the dimming effects of distance on light, and to do that, we must know how far away they are. Distance is among the most difficult of all astronomical measurements. We will return to how it is determined after we have learned more about the stars. For now, we will describe how astronomers specify the apparent brightness of stars.

Around B. There he prepared a catalog of nearly stars that included not only their positions but also estimates of their apparent brightnesses. Hipparchus did not have a telescope or any instrument that could measure apparent brightness accurately, so he simply made estimates with his eyes. He sorted the stars into six brightness categories, each of which he called a magnitude.

He referred to the brightest stars in his catalog as first-magnitudes stars, whereas those so faint he could barely see them were sixth-magnitude stars. During the nineteenth century, astronomers attempted to make the scale more precise by establishing exactly how much the apparent brightness of a sixth-magnitude star differs from that of a first-magnitude star.

Measurements showed that we receive about times more light from a first-magnitude star than from a sixth-magnitude star. Based on this measurement, astronomers then defined an accurate magnitude system in which a difference of five magnitudes corresponds exactly to a brightness ratio of