The birth and evolution of stars. Presentation Topic: The birth and evolution of stars Is it possible to see in the sky how stars are formed?
Slide 2
Stellar evolution is the sequence of changes that a star undergoes during its life, that is, over hundreds of thousands, millions or billions of years while it emits light and heat. Over such enormous periods of time, the changes are quite significant.
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The evolution of a star begins in a giant molecular cloud, also called a stellar cradle. Most of the “empty” space in a galaxy actually contains between 0.1 and 1 molecule per cm³. A molecular cloud has a density of about a million molecules per cm³. The mass of such a cloud exceeds the mass of the Sun by 100,000-10,000,000 times due to its size: from 50 to 300 light years in diameter. While the cloud rotates freely around the center of its home galaxy, nothing happens. However, due to the inhomogeneity of the gravitational field, disturbances may arise in it, leading to local concentrations of mass. Such disturbances cause gravitational collapse of the cloud.
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During collapse, the molecular cloud is divided into parts, forming smaller and smaller clumps. Fragments with a mass less than ~100 solar masses are capable of forming a star. In such formations, the gas heats up as it contracts due to the release of gravitational potential energy, and the cloud becomes a protostar, transforming into a rotating spherical object. Stars in the early stages of their existence are usually hidden from view within a dense cloud of dust and gas. These star-forming cocoons can often be seen silhouetted against the bright radiation of the surrounding gas. Such formations are called Bok globules.
Slide 5
Young low-mass stars (up to three solar masses) approaching the main sequence are completely convective; The convection process covers all areas of the sun. These are essentially protostars, in the center of which nuclear reactions are just beginning, and all radiation occurs mainly due to gravitational compression. While hydrostatic equilibrium has not yet been established, the star's luminosity decreases at a constant effective temperature.
Slide 6
A very small fraction of protostars do not reach temperatures sufficient for thermonuclear fusion reactions. Such stars are called “brown dwarfs”; their mass does not exceed one tenth of the Sun. Such stars die quickly, gradually cooling over several hundred million years. In some of the most massive protostars, the temperature due to strong compression can reach 10 million K, making it possible to synthesize helium from hydrogen. Such a star begins to shine.
Slide 7
The combustion reaction of helium is very sensitive to temperature. Sometimes this leads to great instability. Strong pulsations arise, which ultimately impart sufficient acceleration to the outer layers to be thrown off and turn into a planetary nebula. In the center of the nebula, the bare core of the star remains, in which thermonuclear reactions stop, and as it cools, it turns into a helium white dwarf, usually having a mass of up to 0.5-0.6 solar and a diameter on the order of the diameter of the Earth.
Slide 8
When a star reaches an average size (from 0.4 to 3.4 solar masses) the red giant phase, its core runs out of hydrogen and the reactions of carbon synthesis from helium begin. This process occurs at higher temperatures and therefore the flow of energy from the core increases, which leads to the fact that the outer layers of the star begin to expand. The beginning of carbon synthesis marks a new stage in the life of a star and continues for some time. For a star similar in size to the Sun, this process can take about a billion years.
Slide 9
Young stars with a mass greater than 8 solar masses already have the characteristics of normal stars, since they have gone through all the intermediate stages and were able to achieve such a rate of nuclear reactions that they compensate for energy losses due to radiation while the mass of the hydrostatic core accumulates. For these stars, the outflow of mass and luminosity are so great that they not only stop the collapse of the outer regions of the molecular cloud that have not yet become part of the star, but, on the contrary, push them away. Thus, the mass of the resulting star is noticeably less than the mass of the protostellar cloud. Most likely, this explains the absence in our galaxy of stars larger than about 300 solar masses.
Slide 10
After a star with a mass greater than five times the sun enters the red supergiant stage, its core begins to shrink under the influence of gravity. As compression increases, temperature and density increase, and a new sequence of thermonuclear reactions begins. In such reactions, increasingly heavier elements are synthesized: helium, carbon, oxygen, silicon and iron, which temporarily restrains the collapse of the core. Ultimately, as heavier and heavier elements of the periodic table are formed, iron-56 is synthesized from silicon. At this stage, further thermonuclear fusion becomes impossible since the iron-56 nucleus has a maximum mass defect and the formation of heavier nuclei with the release of energy is impossible. Therefore, when the iron core of a star reaches a certain size, the pressure in it is no longer able to withstand the gravity of the outer layers of the star, and immediate collapse of the core occurs with neutronization of its matter.
Slide 11
The accompanying burst of neutrinos provokes a shock wave. Strong jets of neutrinos and a rotating magnetic field push out much of the star's accumulated material - the so-called seed elements, including iron and lighter elements. The scattering matter is bombarded by neutrons ejected from the nucleus, capturing them and thereby creating a set of elements heavier than iron, including radioactive ones, up to uranium (and possibly even californium). Thus, supernova explosions explain the presence of elements heavier than iron in interstellar matter, which, however, is not the only possible way of their formation, for example, this is demonstrated by technetium stars.
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The blast wave and neutrino jets carry matter away from the dying star into interstellar space. Subsequently, as it cools and moves through space, this supernova material can collide with other space “junk” and possibly participate in the formation of new stars, planets or satellites. The processes occurring during the formation of a supernova are still being studied, and so far there is no clarity on this issue. Also questionable is what actually remains of the original star. However, two options are being considered: neutron stars and black holes.
Slide 13
The Crab Nebula is a gaseous nebula in the constellation Taurus, which is a supernova remnant and a plerion. It became the first astronomical object identified with a historical supernova explosion, recorded by Chinese and Arab astronomers in 1054. Located about 6,500 light-years (2 kpc) from Earth, the nebula has a diameter of 11 light-years (3.4 pc) and is expanding at a speed of about 1,500 kilometers per second. At the center of the nebula is a neutron star, 28-30 km in diameter, which emits pulses of radiation ranging from gamma rays to radio waves. With X-ray and gamma-ray emissions above 30 keV, this pulsar is the strongest persistent source of such radiation in our galaxy.
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Slide 2
The Universe consists of 98% stars. They are also the main element of the galaxy.
“Stars are huge balls of helium and hydrogen, as well as other gases. Gravity pulls them in, and the pressure of the hot gas pushes them out, creating equilibrium. The energy of a star is contained in its core, where helium interacts with hydrogen every second.”
Slide 3
The life path of stars is a complete cycle - birth, growth, a period of relatively quiet activity, agony, death, and resembles the life path of an individual organism.
Astronomers are unable to trace the life of one star from beginning to end. Even the shortest-lived stars exist for millions of years - longer than the life of not only one person, but of all humanity. However, scientists can observe many stars at very different stages of their development - newly born and dying. Based on numerous star portraits, they try to reconstruct the evolutionary path of each star and write its biography.
Slide 4
Hertzsprung-Russell diagram
Slide 5
Star forming regions.
Giant molecular clouds with masses greater than 105 solar masses (more than 6,000 of them are known in the Galaxy)
The Eagle Nebula, 6,000 light years away, is a young open star cluster in the constellation Serpens; the dark areas in the nebula are protostars.
Slide 6
The Orion Nebula is a luminous emission nebula with a greenish tint and is located below Orion's Belt, visible even with the naked eye, 1300 light years away, and a magnitude of 33 light years
Slide 7
Gravitational compression
Compression is a consequence of gravitational instability, Newton's idea.
Jeans later determined minimum dimensions clouds in which spontaneous compression may begin.
There is a fairly effective cooling of the medium: the released gravitational energy goes into infrared radiation that goes into outer space.
Slide 8
Protostar
- As the density of the cloud increases, it becomes opaque to radiation.
- The temperature of the internal regions begins to rise.
- The temperature in the bowels of a protostar reaches the threshold of thermonuclear fusion reactions.
- The compression stops for a while.
Slide 9
- a young star has arrived on the main sequence of the H-R diagram
- the process of burning out hydrogen, the main stellar nuclear fuel, has begun
- compression practically does not occur, and energy reserves no longer change
- a slow change in the chemical composition in its central regions, caused by the conversion of hydrogen into helium
The star goes into a stationary state
Slide 10
Evolution graph of a typical star
Slide 11
when the hydrogen completely burns out, the star leaves the main sequence into the region of giants or, at high masses, supergiants
Giants and supergiants
Slide 12
- star mass< 1,4 массы Солнца: БЕЛЫЙ КАРЛИК
- electrons are shared, forming a degenerate electron gas
- gravitational compression stops
- density reaches several tons per cm3
- still retains T=10^4 K
- gradually cools and slowly contracts (millions of years)
- finally cool down and turn into BLACK Dwarfs
When all the nuclear fuel has burned out, the process of gravitational compression begins.
Slide 13
- White dwarf in a cloud of interstellar dust
- Two young black dwarfs in the constellation Taurus
Slide 14
- star mass > 1.4 solar masses:
- gravitational compression forces are very high
- the density of the substance reaches a million tons per cm3
- enormous energy is released - 10^45 J
- temperature – 10^11 K
- supernova explosion
- most of the star is thrown into outer space at a speed of 1000-5000 km/s
- neutrino streams cool the star's core -
Neutron star
Presentation
Topic: The birth and evolution of stars
Rodkina L. R.
Associate Professor, Department of Electronics, IIBS
VGUES, 2009
The Birth of Stars
Life of a star
White dwarfs and neutron holes
Black holes
Death of the Stars
Goals and objectives
Introduce the action of gravitational forces in the Universe, which lead to the formation of stars.
Consider the process of evolution of stars.
Give the concept of the spatial speed of stars.
Describe the physical nature of stars.
A star is born
A star is born
A star is born
Life of a star
Life of a star
The lifetime of a star depends mainly on its mass. According to theoretical calculations, the mass of a star can vary from 0,08 before 100 solar masses.
The greater the mass of a star, the faster the hydrogen burns, and the heavier elements can be formed during thermonuclear fusion in its depths. At a late stage of evolution, when helium combustion begins in the central part of the star, it leaves the Main Sequence, becoming, depending on its mass, a blue or red giant.
Life of a star
Life of a star
Death of a star
Bibliography:
Shklovsky I. S. Stars: their birth, life and death. - M.: Nauka, Main editorial office of physical and mathematical literature, 1984. - 384 p.
Vladimir Surdin How stars are born - Rubric “Planetarium”, Around the World, No. 2 (2809), February 2008
Control questions
Where do stars come from?
How do they arise?
Since the lifetime of stars is limited, they must arise in a finite time. How could we learn something about this process?
Is it possible to see stars forming in the sky?
Are we witnessing their birth?
Used Books
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The Universe consists of 98% stars. They are also the main element of the galaxy. “Stars are huge balls of helium and hydrogen, as well as other gases. Gravity pulls them in, and the pressure of the hot gas pushes them out, creating equilibrium. The energy of a star is contained in its core, where helium interacts with hydrogen every second.”
The life path of stars is a complete cycle - birth, growth, a period of relatively quiet activity, agony, death, and resembles the life path of an individual organism. Astronomers are unable to trace the life of one star from beginning to end. Even the shortest-lived stars exist for millions of years - longer than the life of not only one person, but of all humanity. However, scientists can observe many stars at very different stages of their development - newly born and dying. Based on numerous star portraits, they try to reconstruct the evolutionary path of each star and write its biography.
Star forming regions. Giant molecular clouds with masses greater than 105 times the mass of the Sun (they are better known in the Galaxy) The Eagle Nebula, 6000 light years from us, a young open star cluster in the constellation Serpens, dark areas in the nebula are protostars
Gravitational compression Compression is a consequence of gravitational instability, Newton's idea. Jeans later determined the minimum size of clouds in which spontaneous compression can begin. There is a fairly effective cooling of the medium: the released gravitational energy goes into infrared radiation that goes into outer space.
Protostar As the density of a cloud increases, it becomes opaque to radiation. The temperature of the internal regions begins to rise. The temperature in the bowels of a protostar reaches the threshold of thermonuclear fusion reactions. The compression stops for a while.
The young star has arrived on the main sequence of the H-R diagram; the process of burning out hydrogen has begun - the main stellar nuclear fuel is practically not compressed, and energy reserves no longer change; a slow change in the chemical composition in its central regions, caused by the conversion of hydrogen into helium; The star enters a stationary state
Star mass
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Content
- The Birth of Stars
- Life of a star
- White dwarfs and neutron holes
- Black holes
- Death of the Stars
- Introduce the action of gravitational forces in the Universe, which lead to the formation of stars.
- Consider the process of evolution of stars.
- Give the concept of the spatial speed of stars.
- Describe the physical nature of stars.
- Space is often called airless space, thinking of it as empty. However, it is not. In interstellar space there is dust and gas, mainly helium and hydrogen, with much more of the latter.
- There are even entire clouds of dust and gas in the Universe that can be compressed under the influence of gravity.
- During the compression process, part of the cloud will heat up and become denser.
- If the mass of the compressed substance is sufficient for nuclear reactions to begin to occur within it during the compression process, then a star emerges from such a cloud.
- Each “newborn” star, depending on its initial mass, occupies a certain place on the Hertzsprung-Russell diagram - a graph on one axis of which the color of the star is plotted, and on the other - its luminosity, i.e. the amount of energy emitted per second.
- The color index of a star is related to the temperature of its surface layers - the lower the temperature, the redder the star, and the greater its color index.
- During the process of evolution, stars change their position on the spectrum-luminosity diagram, moving from one group to another. The star spends most of its life on the Main Sequence. To the right and up from it are located both the youngest stars and stars that have advanced far along their evolutionary path.
- The lifetime of a star depends mainly on its mass. According to theoretical calculations, the mass of a star can vary from 0,08 before 100 solar masses.
- The greater the mass of a star, the faster the hydrogen burns, and the heavier elements can be formed during thermonuclear fusion in its depths. At a late stage of evolution, when helium combustion begins in the central part of the star, it leaves the Main Sequence, becoming, depending on its mass, a blue or red giant.
- But there comes a time when a star is on the verge of a crisis; it can no longer generate the required amount of energy to maintain internal pressure and resist the forces of gravity. The process of uncontrollable compression (collapse) begins.
- As a result of the collapse, stars with enormous density (white dwarfs) are formed. Simultaneously with the formation of a superdense core, the star sheds its outer shell, which turns into a gas cloud - a planetary nebula and gradually dissipates in space.
- A star of greater mass can shrink to a radius of 10 km, turning into a neutron star. One tablespoon of a neutron star weighs 1 billion tons! The final stage in the evolution of an even more massive star is the formation of a black hole. The star contracts to such a size that the second escape velocity becomes equal to the speed of light. In the area of a black hole, space is greatly curved and time slows down.
- The formation of neutron stars and black holes is necessarily associated with a powerful explosion. A bright point appears in the sky, almost as bright as the galaxy in which it flared up. This is a "Supernova". Mentions found in ancient chronicles about the appearance of the brightest stars in the sky are nothing more than evidence of colossal cosmic explosions.
- The star loses its entire outer shell, which, flying away at high speed, dissolves without a trace in the interstellar medium after hundreds of thousands of years, and before that we observe it as an expanding gas nebula.
- For the first 20,000 years, the expansion of the gas shell is accompanied by powerful radio emission. During this time, it is a hot plasma ball that has a magnetic field that holds the high-energy charged particles formed in the Supernova.
- The more time has passed since the explosion, the weaker the radio emission and the lower the temperature of the plasma.
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