A Star Is Born (Part II)



Source: NASA Goddard Photo and Video (


We pick up where we left off with Part I.

One of the characteristics which define stars is also their color. Stars come in a whole variety of colors, from those tinted red to those that have a hint of blue in them, and everything else in between. Although it may seem that a star has a single color, it actually emits a spectrum of various colors, but also radio waves, infrared and ultraviolet light and gamma rays. We are able to determine to composition of the star by analyzing which elements or compounds the star is made of emit or absorb which wavelengths or colors. The color of the star is dependent on the surface temperature.

Speaking of which, we measure the temperature of the star’s surface in units known as the kelvin. For example, dark red stars have a surface temperature of about 2,500 K, bright red ones about 3,500 K, yellow stars, including our Sun, about 5,500 K, blue stars about 10,000 to 50,000 K. The temperature on the surface of the star depends partially on the stars’ mass and it also influences its brightness and color. More precisely, the luminosity of a star is proportional to temperature to the fourth power. If two are equal in size, but one of them is twice as hot as the other, the former star is 16 times as luminous as the latter.

The size of a star is measured by astronomers in terms of the radius of our Sun. Alpha Centauri A has a radius of 1.05 solar radii. They can vary greatly in size, from some neutron star which are only 20 kilometers wide, to supergiants that more than 1000 times larger the the Sun. The size of a star has an affect of its brightness, as luminosity is proportional to radius squared. For example, if two stars have the same surface temperature, and one of them is twice as wide as the other one, the former star would be four times as bright as the latter.

A flock of stars
Source: ESA/Hubble & NASA Acknowledgement: Luca Limatola (


The mass of the star is also represented in terms related to our Sun, therefore, Alpha Centauri A, for example, is 1.08 solar masses. If two stars are similar in mass, that doesn’t necessarily mean that they are similar in size because they can different densities. To illustrate this better, Sirius B has almost the same mass as our Sun, but it is 90,000 times denser, so it’s only one fiftieth its diameter. The mass of a star affects the surface temperature.

Seeing as the stars are electrically charged gaseous balls, they typically induce magnetic fields. The scientists have discovered that the Sun’s magnetic field can become highly concentrated in small areas, which causes the occurrence ranging from sunspots to spectacular eruptions such as flares and coronal mass ejections. However, directly detecting the magnetic fields of other stars can be difficult.

The metallicity of a star measures the amount of elements the star has that are heavier than helium.

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