LIFE CYCLE OF A STAR Stars are formed in nebulae, interstellar clouds of dust and gas (mostly hydrogen).
These stellar nurseries are abundant in the arms of spiral galaxies. In these stellar nurseries, dense parts of these clouds undergo gravitational collapse and compress to form a rotating gas globule. The globule is cooled by emitting radio waves and infrared radiation. It is compressed by gravitational forces and also by shock waves of pressure from supernova or the hot gas released from nearby bright stars. These forces cause the roughly-spherical globule to collapse and rotate.
The process of collapse takes from between 10, 000 to 1, 000, 000 years. A Central Core and a Proto planetary Disk: As the collapse proceeds, the temperature and pressure within the globule increases, as the atoms are in closer proximity. Also, the globule rotates faster and faster. This spinning action causes an increase in centrifugal forces (a radial force on spinning objects) that causes the globule to have a central core and a surrounding flattened disk of dust (called a proto planetary disk or accretion disk).
The central core becomes the star; the proto planetary disk may eventually coalesce into orbiting planets, asteroids, etc. Proto star: The contracting cloud heats up due to friction and forms a glowing proto star; this stage lasts for roughly 50 million years. If there is enough material in the proto star, the gravitational collapse and the heating continue. If there is not enough material in the proto star, one possible outcome is a brown dwarf (a large, not-very-luminous celestial body having a mass between 1028 kg and 84 x 1028 kg).
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A Newborn Star: When a temperature of about 27, 000, 000^A^0 F is reached, nuclear fusion begins. This is the nuclear reaction in which hydrogen atoms are converted to helium atoms plus energy.
This energy (radiation) production prevents further contraction of the star. Young stars emit jets of intense radiation that heat the surrounding matter to the point at which it glows brightly. These narrowly-focused jets can be trillions of miles long and can travel at 500, 000 miles per hour. These jets may be focused by the star’s magnetic field. The proto star is now a stable main sequence star which will remain in this state for about 10 billion years. After that, the hydrogen fuel is depleted and the star begins to die.
Life span: The most massive stars have the shortest lives. Stars that are 25 to 50 times that of the Sun live for only a few million years. Stars like our Sun live for about 10 billion years. Stars less massive than the Sun have even longer life spans. The energy the star gains by fusing these atoms keeps it from collapsing.
If a star is massive enough, it will fuse heavier and heavier atoms — hydrogen to, to carbon, carbon to… until… elements are fused into iron. Fusing iron to form heavier elements actually requires energy, so the star would not gain anything by continuing fusion of iron atoms. Most of the star’s life is spent fusing hydrogen into helium.
Our sun has been doing this for some five billion years, and is expected to continue doing it for another five billion or so years. This hydrogen burning starts from the very center of the star, and moves its way out, leaving a core of helium behind. Low Mass Stars If the star is small enough (much less than the mass of our Sun), it never gets beyond hydrogen burning. This is because its central temperature never gets high enough to start fusing helium into carbon. Once such a star has used up most of its hydrogen, it will begin to cool and collapse into a ‘brown dwarf’. Intermediate Mass Stars Stars with masses close to that of our Sun (up to about five times the mass of our Sun) will experience helium-to-carbon burning in their cores.
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Outside the helium core, hydrogen will continue burning into helium. At this point, the outer layers of the star will expand to conserve energy — the star swells, becoming brighter and cooler. This is called the red giant phase of the star. The red giant loses many of its outer layers because of the radiation coming from the core blows it away. Eventually the star will cool down so much that the carbon burning stops. Such a star will collapse into a white dwarf.
High Mass Stars High mass stars end their lives spectacularly. They, too, go through a stage where they swell up, though they swell even more than their lower-mass counterparts. This stage is called the red supergiant phase. These stars are so large that their central temperature becomes high enough that further burning in their core will occur.
Eventually, they have so many layers, that they may look like an onion — see figure below. This process necessarily ends when the core has been fused into iron. Once this occurs, the core no longer has any resistance to gravity — the core collapses. During this core collapse, the outer layers of the star are blown off in a supernova explosion. The core collapses either into a neutron star or into a black hole. Neutron Stars During the core collapse of the stars with masses between 15 and 30 times that of our Sun, the electrons and neutrons in the core combine into neutrons.
Usually neutrons will decay into a proton and electron quickly; however, when the density of protons and electrons is high enough, it becomes less advantageous for a neutron to decay. This mass of neutrons will collapse as much as they can without violating the ‘no two objects can occupy the same space’ law of physics (the Pauli exclusion principle).
Neutron stars are about 10 km (6 miles! ) in diameter with a mass of about one and a half times that of our Sun. This makes for a huge density! ^aEURc Main sequence stars; these define a curved trend across the centre of the diagram which displays a relationship between mass and luminosity, such that stars with a high luminosity have a high effective temperature. ^aEURc Red giants are stars which are cooler, but more luminous than stars on the main trend ^aeurc” these are thought to be either dead stars or proto stars. ^aEURc Super giants are similarly stars which are more luminous, but much cooler than main trend stars.
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^aEURc White Dwarfs are stars which are hotter and less luminous than main trend stars and are thought to be dead stars radiating away their energy. Indirect Measurement The key features of stars which are of interest to astronomers are their mass, their luminosity, their surface temperature and the distance they are away from us. A degree of luminosity of an object in the sky (galaxy; star; glowing clouds; planet) can be represented by its apparent magnitude – a measure of how bright it actually appears as seen by the telescope or other measuring device. This magnitude is a function of 1) the intrinsic brightness which varies as a function of size, mass, and spectral type (related to star’s surface temperature) and 2) its distance from Earth. (Magnitude as applied to a galaxy, which seldom shows many individual stars unless they are close [generally less than a billion years away], is an integrated value for the unresolved composite of glowing stars and gases within it. ) The brightness of a star can be measured photometrically (at some arbitrary wavelength range) and assigned a luminosity L (radiant flux).
For two stars (a and b) whose luminosities have been determined, this relationship holds: La/Lb = (2. 512) mb – ma from which can be derived: mb – ma = 2. 5 log (La/Lb) To establish a numerical scale, some reference star (s) must be assigned an arbitrary value. Initially, the star chosen, Polaris, was rated at +2.
0 but when it was later found to be a variable star, others were selected to be the 0 reference value for m. The magnitude scale ranges from -m (very bright) to +m (increasingly faint) values. The more positive the number, the fainter is the object (planet; star; galaxy); very distant galaxies, even though these may be extremely luminous, could have large positive apparent magnitudes because of the 1/r 2 decrease in brightness with increasing distance. The Sun has the value – 26. 5; the full Moon is -12. 5; Venus is -4.
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4; the naked eye can see stars brighter than + 7; Pluto has a magnitude of +15; Earth-based telescopes can pick out stars visually with magnitudes down to ~+ 20 (faintest) and with CCD integrators to about +28, and the HST to about +30. Thus, the trend in these values is from decreasing negativity to increasing positivity as the objects get ever less luminous as observed through a telescope. Each change in magnitude by 1 unit represents an increase / decrease in apparent brightness of 2. 512; a jump of 3 units towards decreasing luminosity, say from magnitude +4 to +7, results in a (2. 512) 3 = 15. 87 decrease in brightness (the formula for this is derivable from the above equations, such that the ratio of luminosities is given by this expression: 10 (0.
4) (mb – ma).
Below is a simple linear graph that shows various astronomical objects plotted on the apparent magnitude scale: From Nick Strobel’s Astronomy Notes. absolute magnitude (M) is the apparent magnitude (m) a star would have if it were relocated to a standard distance from Earth. Apparent magnitude can be converted to absolute magnitude by calculating what the star’s or galaxy’s luminosity would appear to be if it were conceived as being moved to a reference distance of 10 parsecs (10 x 3. 26 light years) from Earth.
The formula for this is: M = m + 5 – 5 log 10 r, where r is the actual distance (in parsecs) of the star from Earth. Both positive and negative values for M are possible. The procedure envisions all stars of varying intrinsic brightness^aEURTMs and at varying distances from Earth throughout the Cosmos as having been arbitrarily relocated at a single common distance away from the Earth. Both luminosity and magnitude are related to a star’s mass (which is best determined by applying Newton’s Laws of motion to binary stars [a pair; see below for a discussion of binaries]).
The graph below, made from data in which mass is determined by gravitational effects, expresses this relationship; in the plot both mass and luminosity are referenced to the Sun (note that the numbers are plotted in logarithmic units on both axes): There is a relationship between absolute magnitude (here given by L for luminosity) and mass (given by the conventional letter M; which accounts for replacing the absolute magnitude M with L).
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Here is one expression: In the above, both L and M for a given star are ratio ed to the values determined for the Sun. Note the two different power exponents. It seems that some stars obey a fourth power, others a 3, and a few are just the square of the mass. The most general expression in use is given as L = M 3. 5. There are relatively few stars with mass greater 50 times the Sun.
Very rarely, we can find a star approaching 100 s solar mass, but these are so short-lived that nearly all created before the last million years have exploded, with their mass being highly dispersed, and thus ceasing to send detectable radiation. If the Sun were envisioned as displaced outward to a distance of 32. 6 l. y. , its apparent magnitude as seen from Earth would be -26. 5; its absolute magnitude would be changed to +4.
85. A quasar, which is commonly brighter than a galaxy, has an absolute brightness of – 27 (note that in the absolute scale increasingly negative values denote increasing intrinsic brightness).
The illustration below gives the absolute magnitudes (vertical axis) as a function of temperature (horizontal axis) for a number of stars with popular names; note the similarity of the color bars (which express the visual colors of the stars as seen through a telescope) to the brightness range – this is essentially a preview version of the standard H-D diagram, shown and discussed on this page beginning twelve figures below this, which serves as a plot of the different types of stars and an inferred history of a star of given size (mass): In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength.
Radiation is red shifted when its wavelength increases, and is blue shifted when its wavelength decreases. Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. Spectral line shifts are used to study the motions of binary stars. We will treat stellar spectroscopy in detail on page 20-7 As a preview, the spectral method can be illustrated by looking at a pair of spectral strips for two similar stars that are mutually orbiting: Bright lines for hydrogen appear in the top and bottom (dark background) strips. This fixes a reference location for excited hydrogen in the rest state. The two center spectral strips include the same hydrogen lines, the first strip acquired from one and the second the other star.
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Note that the lines in one have moved to the left and the other to the right of the reference lines position. The spectrum on the bottom center has been blue shifted (see page 20-9) towards shorter wavelengths; the spectrum at the top center has been red shifted towards longer wavelengths. This is explained thusly: The bottom star is in motion towards the observing system on Earth whereas the top star is moving away from the telescope. This would occur when the two stars are aligned sideways to the line of sight and are moving in opposite directions around a common center of gravity.
Both star classification and evolution can be summarized in a graph like chart that consists of a plot of luminosity (vertical axis) versus star surface temperature which is expressed also by (correlated with) the star’s visual color (note also the Spectral Type designations at the top).
This is known as the Hertzsprung-Russell (H-R) Diagram. Mass densities are shown as numbers on the the central line that defines the Main Sequence (M. S. ) of stars. Most known stars lie along this line; they describe a stage in which a star reaches some fixed size and mass and commences burning of most of its hydrogen before changing to some other star type off the sequence.
Star types, which are defined on the basis of stellar surface temperatures (page 20-7), are shown by the letters (O, B, … etc. ) assigned to each group and evolutionary pathways for some are indicated. This particular plot also shows along the right ordinate the total time that Main Sequence stars of different masses spend on that sequence before evolving along the several principal pathways (see below); as far as we now know, stars do not completely vanish, but survive as dwarfs or Black Holes (but the latter in principle can disappear by evaporation as Hawking radiation).
Temperatures were obtained using Chandra X-ray data.
BIBLIOGRAPHY web main p 1… org / news center / news desk / archive /releases/1999/20/map. gs fc. nasa.
gov / m uni / uni 101 stars. htm.