How do you calculate luminosity of a star

Apparent magnitude, on the other hand, is a measure of brightness when the star is seen from Earth - hence, it takes into account the distance between the star and the Earth. You can find it with the apparent magnitude calculator, using the following equation:. The absolute magnitude is defined as the apparent magnitude of an object seen from the distance of 10 parsecs. Let's analyze Sun with this luminosity calculator to investigate its absolute and apparent magnitude.

Input the radius and temperature of the Sun into the calculator. The luminosity calculator will automatically find the luminosity of the Sun. It is equal to 3. Embed Share via. Luminosity Calculator By Bogna Szyk. Table of contents: What is luminosity? Luminosity equation Absolute and apparent magnitude Calculating luminosity: an example. That is, the searchlight is more luminous. If that searchlight is 5 miles 8 kilometers away from you, however, it will not be as bright because light intensity decreases with distance squared.

A searchlight 5 miles from you may look as bright as a penlight 6 inches 15 centimeters away from you. The same is true for stars. Astronomers professional or amateur can measure a star's brightness the amount of light it puts out by using a photometer or charge-coupled device CCD on the end of a telescope. If they know the star's brightness and the distance to the star, they can calculate the star's luminosity:.

Luminosity is also related to a star's size. What is radio astronomy? What is a pulsar? Luminosity , L , is a measure of the total amount of energy radiated by a star or other celestial object per second. This is therefore the power output of a star. A star's power output across all wavelengths is called its bolometric luminosity. Astronomers in practice also measure an object's luminosity in specific wavebands so that we can discuss an object's X-ray or visible luminosities for example.

This is also used to measure a star's colour as described on the next page. Our Sun has a luminosity of 3. This approach is convenient as the luminosity of stars varies over a huge range from less than 10 -4 to about 10 6 times that of the Sun so an order of magnitude ratio is often sufficient.

As we have seen in the section on spectroscopy, we can approximate the behaviour of stars as black body radiators. Fundamentally there are just two key properties - the effective temperature, T eff and the size of the star, its radius, R. Let us look briefly at each of these:. Temperature : A black body radiates power at a rate related to its temperature - the hotter the black body, the greater its power output per unit surface area.

An incandescent or filament light bulb is an everyday example. As it gets hotter it gets brighter and emits more energy from its surface. While many stars have a consistent brightness, there are more than , known and cataloged variable stars. Even our own sun is variable, varying its energy output by about 0. Variable stars are either intrinsic meaning their luminosity changes due to features such as expansion, contraction, eruption or pulsation or extrinsic meaning that a star or planet passes in front of the star and blocks the light, or that the change is due to stellar rotation.

Stars can also change in luminosity over time. The North Star or Polaris, for example, could have been as much as 4. A study noted that the star dimmed for the past few decades, but then drastically brightened again. Polaris is part of the class of Cepheid variables, which are extremely luminous stars that have short pulsation periods.

The variations in the luminosity allow astronomers to calculate how far away these Cepheids are, making them useful "measuring sticks" if the stars are embedded in galaxies or nebulae. Other types of intrinsic variable stars include cataclysmic variables which brighten due to outbursts, such as during supernovae explosions or eruptic variables whose brightness varies during eruptions on the surface, or combinations with interstellar matter.

Extrinsic variables include eclipsing binary stars and rotating stars such as pulsars, the cores of supernova whose electromagnetic radiation is only visible when the beam is directed at Earth. Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community space. Elizabeth Howell is a contributing writer for Space.