Classifying Stars - the Hertzsprung-Russell Diagram

There room a couple of hundred exchange rate stars in ours galaxy, the Milky method and billions that galaxies in the Universe. One important technique in science is to try and kind or classify things right into groups and seek out trends or patterns. Astronomers carry out this with stars.

So much we have disputed the luminosity and also colour or reliable temperature of stars. These can be plotted to form what is among the most beneficial plots for stellar astronomy, the Hertzsprung-Russell (or H-R) diagram. The is named after the Danish and also American astronomers that independently arisen versions of the chart in the early Twentieth Century.

In one H-R diagram the luminosity or energy output that a star is plotted on the vertical axis. This have the right to be expressed together a ratio of the star"s luminosity to that of the Sun; L*/Lsun. Astronomers likewise use the historical ide of magnitude as a measure of a star"s luminosity. Absolute magnitude is merely a measure up of just how bright a star would show up if 10 parsecs distant and also thus permits stars come be simply compared. Simply to confuse things, the reduced or more negative the magnitude, the brighter the star. By meaning a star of magnitude 1 is 100 × brighter than one of magnitude 6. Ours Sun has an absolute magnitude that + 4.8.


Possible axes for a Hertzsprung-Russell Diagram.

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Note exactly how the temperature scale is reversed on the horizontal axis. Likewise take care if using magnitude to work upwards to an adverse values.

The reliable temperature that a star is plotted ~ above the horizontal axis of an H-R chart . One quirk below is that the temperature is plotted in reverse order, v high temperature (around 30,000 - 40,000 K) ~ above the left and also the cooler temperature (around 2,500 K) ~ above the right. In practice astronomers actually measure a quantity referred to as colour index the is just the difference in the size of a star as soon as measured with two different coloured filters. Stars with a negative colour index are bluish whilst cooler orange or red stars have a optimistic colour index.

The third feasible scale because that the horizontal axis is a star"s spectral class. By dividing the light from a star through a spectrograph the spectrum have the right to be recorded and also analysed. Stars of comparable size, temperature, composition and other nature have similar spectra and are classified right into the same spectral class. The main spectral classes because that stars variety from O (the hottest) v B, A, F, G, K and M (coolest). Our sunlight is a G-class star. By to compare the spectra of one unknown star with spectra of selected standard reference stars a riches of information, including its colour or reliable temperature deserve to be determined.

If we currently plot a Hertzsprung-Russell diagram because that a few thousand nearest or brightest stars we watch the following:


As we deserve to see, stars execute not show up randomly on the plot but show up to be group in 4 main regions. This is highly far-ranging as it argues that there might be part relationship in between the luminosity and also temperature that a star. Whilst no surprising (indeed we have already seen the a hotter star emits much more energy every unit surface area than a cooler star) the connection is complicated by the visibility of these 4 groups. Permit us research these an ext closely.

Most stars seem to autumn into group A. It mirrors a general trend indigenous cool, dim stars in the reduced right edge up come hot, exceptionally bright stars in the height left corner which fits in through our intended relationship in between temperature and also luminosity. This group is referred to as the Main Sequence for this reason stars uncovered on that are key sequence stars. Our sunlight is one such example. Others encompass α Cen, Altair, Sirius, Achernar and also Barnard"s Star.

Stars in group B are greatly 6,000 K or cooler yet much more luminous than main sequence stars that the exact same temperature. How deserve to this be? The reason is the these stars space much bigger than main sequence stars. Although they emit the exact same amount of energy per square metre as key sequence stars they have have much greater surface area (area ∝ radius2) the complete energy emitted is thus much greater. These stars are referred to as giants. Examples incorporate Aldebaran and Mira.

The stars in group C space even an ext luminous 보다 the giants. These space the supergiants, the largest of stars with incredibly high luminosities. A red supergiant such together Betelgeuse would extend past the orbit of Jupiter if it changed the sunlight in our solar system.

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The final group that interest space those stars in group D. Indigenous their place on the H-R diagram we check out that they are really hot yet an extremely dim. Although they emit big amounts of power per square metre they have low luminosity which suggests that they must because of this be very small. Group D stars space in fact known as white dwarfs. Sirius B and also Procyon B room examples. White dwarfs are much smaller than main sequence stars and are around the size of Earth. The diagram listed below shows the main groups labelled along with example stars in each group.


Having established the presence of different species of stars based upon measurable properties in the following section we will explore some of their characteristics and the resources of energy in the stars.