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    astronomers have determined the surface temperature of stars by studying their colors. what color emission represents stars with the hottest temperatures?


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    Jupiter is about 5.2 astronomical units (AU) from the Sun. According to Kepler's Third Law (simplified as P^2 = a^3, where "P" is the period and "a" is the average distance from the Sun), about how long does it take Jupiter to complete one revolution around the Sun?

    140.6 years

    5.2 years

    3 years

    11.86 years


    Source : quizizz.com


    210,000 light years away, in the neighboring galaxy known as the Small Magellanic Cloud, stars are being formed at a rapid rate. New blue stars burning at very high temperatures send out fierce radiation, burning away some of the dense material surrounding them. The remaining dense globules form nurseries for more stars.

    Stars great and small, and their life cycles

    A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K.

    Stars are fuelled by the nuclear fusion reactions at their core. There is a dynamic equilibrium maintained throughout the star’s life between the expanding heat of the reactive core and gravitational forces holding the star together. Fusion produces extremely high energy. Fusion releases some of the energy that binds the particles of the nucleus together, unleashing remarkable power.

    A star does not stay the same color throughout its lifecycle, since the surface temperature alters depending on the type of fusion reaction fuelling the star at the time. Depending on the initial mass of the star, it will evolve along the lines of one of three main star types: low-mass stars, intermediate-mass stars (like our sun) and high-mass stars.

    A cross-section through the sun, showing the hot core where fusion reactions fuel the radiation that eventually reaches us as sunlight, the radiative and convective zones carrying light and heat out of the core, the photosphere that comprises the sun’s surface, and the corona, the emissions we know as the solar wind.

    The Cat`s Eye nebula (NGC 6543) is evidence of successive episodes of a star exploding and shedding its outer layers as its core collapses over time.

    Once the nebula fades, the core is called a white dwarf, and has a temperature of 100,000 K. This cools slowly, over billions of years, to become a black dwarf too faint to detect. This is the end of the star’s life.

    Stars far larger than our sun, with a higher mass, become increasingly massive and dense as nuclear fusion creates heavier and heavier chemical elements within them, and their gravity increases. Some explode in supernovae, leaving blisteringly colorful nebulae to mark their passing – at the same time providing material for possible future generations of stars. This image shows Cassiopeia A, a supernova remnant shown in enhanced color, blasted material surrounding the dead neutron star at the center. Red detail is sourced from infrared images, yellow from visible light, and green and blue from x-ray data.

    Images such as this one, from the European Space Agency’s Faint Object Camera (FOC) inside the Hubble Space Telescope, have allowed astronomers to examine the chemical signatures of 43 blue stragglers in the globular cluster 47 Tucanae. This cluster is around 15,000 light years away, and previous images did not provide the resolution to study individual young stars.

    There are two theories for the formation of new stars within a globular cluster: they may be formed by collisions between stars, or by siphoning of material from neighboring stars “captured” by gravity as the stars pass close to each other.

    By examining the light emitted by blue stragglers, astronomers have established that they have less carbon and oxygen than their neighbors. This supports the theory that the new stars form by sucking in material from their partners as they spin around each other in a binary system.


    Nebulae form brilliantly colored spectacles, a phenomenon that becomes increasingly breathtaking as the quality of telescope and spacecraft images improves.

    As we have already seen, a nebula can form in the wake of a star, either the supernova of a high-mass star, or the gas shell of an intermediate-mass star ejected when it becomes a white dwarf. The second type is known as a planetary nebula; early astronomers thought that the shells resembled the discs of planets.

    Arab and Chinese astronomers first recorded the Crab nebula in 1054, when its light first reached the earth, in the constellation of Taurus. At its center are two stars, one a pulsar, a neutron star emitting regular pulses of electromagnetic radiation including radio and x-ray wavelengths.

    The beautiful colors of the Orion Nebula (M42, NGC 1976) are an example of cosmic gas clouds radiating light due to excitation by radiation from nearby stars.

    Telescopes such as the Hubble Space Telescope have increased our knowledge of the universe by providing us with a view through the entire electromagnetic spectral range, without the distortion of the earth’s atmosphere.

    Invisible stars and dark matter

    Many objects in the universe emit electromagnetic radiation that does not fall in the visible spectrum. We can study these objects by measuring the microwaves, x-ray and gamma radiation, and radio waves that they emit.

    This black hole located in the Centaurus galaxy illustrates their gravitational pull: the jet of material flowing into the center is 13,000 light years long and is traveling at half the speed of light. Again, this image is a fusion of visible light and x-ray radiation data.


    Source : www.webexhibits.org

    Lecture 8: How Hot is a Star?

    Astronomy 162: Professor Barbara Ryden

    Wednesday, January 15


    Key Concepts

    (1) A star's surface temperature can be determined from its spectrum.

    Using the above formula, we can compute the temperature of a star's photosphere from the wavelength at which it emits the maximum amount of light.

    The temperature of a star's photosphere can also be deduced from its color. Cool stars (such as Betelgeuse, which has a surface temperature of T = 3500 Kelvin) emit more red and orange light than blue and violet light. Thus, cool stars are red. Hot stars (such as Rigel, which has a surface temperature of T = 15,000 Kelvin) emit more blue and violet light than red and orange light. Thus, hot stars are blue.

    One way of classifying stars is by their temperature; stellar temperatures run from about 2500 Kelvin to about 50,000 Kelvin. Another way of classifying stars is by using the notorious OBAFGKM spectral classes. They are `notorious' because the ordering of the spectral classes, from ``O'' to ``B'' to ``A'' to ``F'' to ``G'' to ``K'' to ``M'', doesn't follow in logical alphabetical order. Initially, in the 19th century, the spectral classes represented a purely empirical method of sorting stellar spectra, based on the strength of their hydrogen absorption lines.

    The OBAFGKM spectral sequence has recently been extended to include class L (objects with temperature around 2000 Kelvin) and class T (with temperature less than 1300 Kelvin). Objects of spectral type L and T are not (technically speaking) stars at all, since they are not hot enough for fusion to occur in their cores. They are called ``brown dwarfs'' rather than ``stars''.

    (2) The Hertzsprung-Russell diagram sorts stars by luminosity and temperature.

    Another way of classifying stars is by LUMINOSITY.

    Make a plot - temperature on the horizontal axis (hot stars to the left; cool stars to the right) and luminosity on the vertical axis (dim stars at the bottom; bright stars at the top). This plot is called the Hertzsprung-Russell diagram (or H-R diagram, for short) after its inventors. An example of a Hertzsprung-Russell diagram is given below.

    H-R diagrams are very useful tools, so I'll be referring to them throughout the entire first half of the course.

    (3) Most stars are on the main sequence of the Hertzsprung-Russell diagram.

    About 90% of all stars are on a narrow diagonal band running from the upper left corner of the H-R diagram (hot, luminous stars) to the lower right corner (cool, dim stars). This diagonal band is called the MAIN SEQUENCE.

    The Sun is on the main sequence.

    Giants are more luminous than a main sequence star of the same temperature. Giants tend to be relatively cool (T < 6000 Kelvin) but luminous (L = 100 to 1000 Lsun). Giants are rare, but easy to detect, because of their high luminosity.

    Supergiants are even more luminous than giants. Supergiants can have any temperature, but they are always VERY luminous, with L = 100,000 to 1,000,000 Lsun. Supergiants are VERY rare, but are VERY easy to detect, because of their VERY high luminosity.

    White Dwarfs are less luminous than a main sequence star of the same temperature. They are called WHITE dwarfs because they are fairly hot; white-hot, in fact, with temperatures of T > 5000 Kelvin. The are low in luminosity, with L = 0.0001 to 0.01 Lsun. White dwarf are fairly common, but they are difficult to detect, because of their low luminosity.

    Take a random sample of 1,000,000 stars from our galaxy. In this sample, you will find, on average:

    Updated: 2003 Jan 15

    Copyright � 2003, Barbara Ryden

    Lecture 8: How Hot is a Star?

    Source : www.astronomy.ohio-state.edu

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