Astronomy - Measuring the Stars
So far we have discussed the luminosity and colour or effective temperature of stars. This can be expressed as a ratio of the star's luminosity to that of the Sun; . In astronomy, stellar classification is the classification of stars based on their spectral . Main-sequence stars vary in surface temperature from approximately 2, to though by the time the first Hertzsprung–Russell diagram was formulated (by .. O-type spectra formerly were defined by the ratio of the strength of the He II. As we have seen, stars are not all the same color because they do not all have identical temperatures. To define color precisely, astronomers have devised.
One also needs to know the distance to the observed objects i. Color distortion including reddening and extinction obscuration are also apparent in stars having significant circumstellar dust. The ideal of direct comparison of theoretical predictions of stellar evolution to observations thus has additional uncertainties incurred in the conversions between theoretical quantities and observations. Interpretation[ edit ] An HR diagram with the instability strip and its components highlighted.
Most of the stars occupy the region in the diagram along the line called the main sequence. During the stage of their lives in which stars are found on the main sequence line, they are fusing hydrogen in their cores. The next concentration of stars is on the horizontal branch helium fusion in the core and hydrogen burning in a shell surrounding the core.
RR Lyrae variable stars can be found in the left of this gap on a section of the diagram called the instability strip.
Star Colors Explained – One Minute Astronomer
Cepheid variables also fall on the instability strip, at higher luminosities. The H-R diagram can be used by scientists to roughly measure how far away a star cluster or galaxy is from Earth.
This can be done by comparing the apparent magnitudes of the stars in the cluster to the absolute magnitudes of stars with known distances or of model stars. The observed group is then shifted in the vertical direction, until the two main sequences overlap.
The difference in magnitude that was bridged in order to match the two groups is called the distance modulus and is a direct measure for the distance ignoring extinction.
This technique is known as main sequence fitting and is a type of spectroscopic parallax. Not only the turn-off in the main sequence can be used, but also the tip of the red giant branch stars. Contemplation of the diagram led astronomers to speculate that it might demonstrate stellar evolutionthe main suggestion being that stars collapsed from red giants to dwarf stars, then moving down along the line of the main sequence in the course of their lifetimes.
Stars were thought therefore to radiate energy by converting gravitational energy into radiation through the Kelvin—Helmholtz mechanism. This mechanism resulted in an age for the Sun of only tens of millions of years, creating a conflict over the age of the Solar System between astronomers, and biologists and geologists who had evidence that the Earth was far older than that.
This conflict was only resolved in the s when nuclear fusion was identified as the source of stellar energy. Following Russell's presentation of the diagram to a meeting of the Royal Astronomical Society inArthur Eddington was inspired to use it as a basis for developing ideas on stellar physics. Inin his book The Internal Constitution of the Stars he explained the physics of how stars fit on the diagram.
This was a particularly remarkable jump of insight, since at that time the source of a star's energy was still unsolved, thermonuclear energy had not been proven to exist, and even that stars are largely composed of hydrogen see metallicityhad not yet been discovered.
O1 to M9 are the hottest to coldest stars. In the MK system, a luminosity class is added using Roman numerals. There is also the Hertzsprung-Russell diagramwhich relates stellar classification to absolute magnitude i.
The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude expressed as Mv to place them on a 2-dimensional chart see below. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses.
At the lower end, K and M type stars orange and red dwarfs tend to be cooler ranging from to Kmeasuring 0.
Star Colors Explained
Stars also go through an evolutionary life cycleduring which time their sizes, temperatures and colors change. For example, when our Sun exhausts all the hydrogen in its the core, it will become unstable and collapse under its own weight. This will cause the core to heat up and get denser, causing the Sun to grow in size. At this point, it will have left its Main Sequence phase and entered into the Red Giant Phase of its life, which as the name would suggest will be characterized by expansion and it becoming a deep red.
When this happens, it is theorized that our Sun will expand to encompass the orbits of Mercury and even Venus. Earth, if it survives this expansion, will be so close that it will be rendered uninhabitable.Stars: Crash Course Astronomy #26
When our Sun then reaches its post-Red Giant Phase, the Sun will begin to eject mass, leaving an exposed core known as a white dwarf. This remnant will survive for trillions of years before fading to black. This is believed to be the case with all stars that have between 0. The situation is slightly different when it comes to low mass stars i. It is believed that these stars can remain in their Main Sequence for some six to twelve trillion years and will not experience a Red Giant Phase. However, they will gradually increase in both temperature and luminosity, and will exist for several hundred billion more years before they eventually collapse into a white dwarf.
On the other hand, supergiant stars up to Solar Masses or more have so much mass in their cores that they will likely experience helium ignition as soon as they exhaust their supplies of hydrogen. As such, they will likely not survive to become Red Supergiants, and will instead end their lives in a massive supernova. To break it all down, stars vary in color depending on their chemical compositions, their respective sizes and their temperatures.
Over time, as these characteristics change as a result of them spending their fuel many will darken and become redder, while others will explode magnificently. The more stars observe, the more we come to know about our Universe and its long, long history!