Why is it difficult to specify the diameter of the galaxy

The Milky Way is about 1,,,,,, km about , light years or about 30 kpc across. The Sun does not lie near the center of our Galaxy. It lies about 8 kpc from the center on what is known as the Orion Arm of the Milky Way. Parallaxes give us distances to stars up to perhaps a few thousand light years.

Beyond that distance, parallaxes are so small than they cannot be measured with contemporary instruments. Astronomers use more indirect methods beyond a few thousand light years. Proper motions: All stars move across the sky, but only for nearby stars are these motions perceivable, and even then it takes decades or centuries to measure. Statistically, stars move at about the same rate; therefore, the stars that appear to have larger motions are nearer. By measuring the motions of a large number of stars of a given class, we can estimate their average distance from their average motion.

Moving clusters: Clusters of stars, such as the Pleiades and Hyades star clusters, travel together. Analyzing the apparent motion of the cluster can give us the distance to it. Interstellar lines: The space between stars is not empty, but contains a sparse distribution of gas. Sometimes this leaves absorption lines in the spectrum we observe from stars that lie beyond the interstellar gas.

Absorption lines are colors missing in a continuous spectrum because of their absorption by atoms or ions. The spectrum is the array of colors or wavelengths that is obtained when light is dispersed. The further a star is, the more absorption will be observed, since the light has passed through more of the interstellar medium.

Inverse-square law: The apparent brightness or magnitude of a star depends both on its intrinsic brightness or luminosity how bright the star actually is rather than how bright it seems and its distance from us. The inverse-square law says that the flux from a luminous object decreases as the square of its distance.

If we know the luminosity of a star for instance, we have a measured parallax for one star of the same type and know that others of the same type will have similar luminosities , we can measure its apparent brightness and then solve for its distance. There are several variations on this, many of which are used to measure distances to stars in other galaxies. Period-luminosity relation: Some stars are regular pulsators, meaning their intensity changes periodically. The physics of their pulsations is such that the period of one oscillation is related to the luminosity of the star.

All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy. Our best estimates tell us that the Milky Way is made up of approximately billion stars. These stars form a large disk whose diameter is about , light years. Our Solar System is about 25, light years away from the center of our galaxy — we live in the suburbs of our galaxy. It takes million years for our Sun and the solar system to go all the way around the center of the Milky Way.

We can only take pictures of the Milky Way from inside the galaxy, which means we don't have an image of the Milky Way as a whole. Why do we think it is a barred spiral galaxy, then? There are several clues. The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky and, as mentioned above, is how the Milky Way got its name.

This band of stars can be seen with the naked eye in places with dark night skies. That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat.

The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. Interestingly enough, the most and least massive galaxies are ellipticals. On average, irregular galaxies have less mass than spirals. This single number tells us roughly what kind of stars make up most of the luminous population of the galaxy, and it also tells us whether a lot of dark matter is present.

For stars like the Sun, the mass-to-light ratio is 1 by our definition. Galaxies are not, of course, composed entirely of stars that are identical to the Sun. The overwhelming majority of stars are less massive and less luminous than the Sun, and usually these stars contribute most of the mass of a system without accounting for very much light. The mass-to-light ratio for low-mass stars is greater than 1 you can verify this using the data in Table 2 of The H-R Diagram.

Galaxies in which star formation is still occurring have many massive stars, and their mass-to-light ratios are usually in the range of 1 to Galaxies consisting mostly of an older stellar population, such as ellipticals, in which the massive stars have already completed their evolution and have ceased to shine, have mass-to-light ratios of 10 to But these figures refer only to the inner, conspicuous parts of galaxies Figure 1.

In The Milky Way Galaxy and above, we discussed the evidence for dark matter in the outer regions of our own Galaxy, extending much farther from the galactic center than do the bright stars and gas. Recent measurements of the rotation speeds of the outer parts of nearby galaxies, such as the Andromeda galaxy we discussed earlier, suggest that they too have extended distributions of dark matter around the visible disk of stars and dust.

This largely invisible matter adds to the mass of the galaxy while contributing nothing to its luminosity, thus increasing the mass-to-light ratio. If dark invisible matter is present in a galaxy, its mass-to-light ratio can be as high as The two different mass-to-light ratios measured for various types of galaxies are given in Table 1. Figure 1: M, the Pinwheel Galaxy.

This galaxy is a face-on spiral at a distance of 21 million light-years. M is almost twice the diameter of the Milky Way, and it contains at least 1 trillion stars. Kuntz Johns Hopkins University , F.