Stellar Physics

Could you give some information on Black Holes ? (Saurabh Chakote)

Some black holes (primordial black holes) were formed in the Big Bang. These are microscopic black holes which have all evaporated by now.There are several ways in which black holes are formed now all of which involve the collapse of a mass more than about 4 times the solar mass. This concentration of mass can arise in following ways:

1) Two white dwarfs in a binary system can merge.

2) Enough mass can be left behind after a supernova explosion.

3) Two neutron stars can merge incidentally giving rise to a gamma ray burster.

It is now believed that the center of every galaxy contains a massive black hole with a mass of several million solar masses which may have formed during the first stages of the galaxy formation. Because the density of matter is inversely proportional to the square of the total mass, the actual density in these giant black holes can be very low.   Top

What causes a black hole ? (Sanjay Amin)

If the mass remaining after a supernova explosion is greater than about four solar masses, the central core collapses under the force of gravity and there is no force strong enough to keep it from continuing to collapse into a black hole.   Top

What is a white dwarf ? What is supernova ?

There two different types of supernovae. A Type I supernova arises when two white dwarfs collide. The resultant explosion can be visible over huge distances and, because all Type I supernovae have essentially the same energy, can be used to measure the distance to the far galaxies in which they occur. A Type II supernova is the explosive death of a massive star.During most of a star's life, its energy comes from hydrogen fusion in the core. Eventually the hydrogen is used up and, depending on the size of the star, progressively heavier elements (helium, carbon and so on) begin fusion. Finally, for the most massive stars, the temperature in the core becomes hot enough for iron atoms to fuse. Whereas the previous stages released energy, iron fusion absorbs energy causing the core to collapse. With no support the outer shells fall onto the core at a substantial fraction of the speed of light followed by a massive explosion releasing huge amounts of energy, more energy in a fraction of a second than the Sun emits in its entire life. Depending on the mass that is left behind in the core, different objects can be formed. If the mass is less than the Chandrasekhar limit of 1.4 Solar masses, a white dwarf is formed. These stars are still hot and continue to radiate for millions of years but eventually will cool down to become cold dark lumps (black dwarfs) sitting in space.   Top

The nebula condenses during the birth of a star. Further, the gas condenses due to the effects of gravity. Where does the gravity comes from ?

Any mass will attract any other mass due to gravitational attraction. In a nebula, a central density concentration forms which will attract material from further out in nebula due to its gravitational force.   Top

Could you give some information about birth and death of a star ? (Saurabh Chakote)

Some of the links about birth and death of a star are :

The Life and Death of Stars

The Life Cycle of Stars

The Lives of Stars

HST Exhibit - Stars Live & Die

Enchanted Learning   Top


How is the inward pull of gravity overwhelmed in a neutron star ? (R T Mangala Cheliyan)

Gravity is always attractive and, unless there were an opposing force, would cause the neutron star, or any other object to collapse into a black hole. In a neutron star the opposing force is simply that the neutrons cannot be forced any closer together.   Top

What is Stellar Spectroscopy ? What are its uses and applications ? (Varalakshmi)

We can see only the light from the star and not other forms of its radiation. Just as the sun's light is split into seven colours with a prism, the star light also is split. However, we must make special efforts to see these split colours of the star, generally referred to as the spectrum. By using diffraction grating, which has over 500 groves engraved in a mm, the
dispersion can be improved. The colours of the stars are indicators of their temperatures. The dark lines that are superposed on the seven colours are signatures of the elements that are present in a star. The thickness of these lines also indicate the evolutionary stage of the star. Therefore, stellar spectroscopy holds the key to understand the internal structure of the stars.
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Why do stars jump suddenly from place toplace ?

This is the phenomenon of twinkling. If you look at a star through a telescope, the point image of the star appears to change position very quickly. Of course, the angular separation between the successive positions is small, as small as 3 arc seconds (arc second is 1/3600th of a degree). This happens because of the turbulent atmosphere (of the earth). The same thing happens to the sun, moon and planets. Owing to their proximity, they look like discs (they are not point objects). However, the edges of these, viewed through telescope, display the effect.   Top

Can we determine the exact status of a particular star as of today ? If yes, how ? (Nisha Giri)

Because no information can be transmitted at a speed greater than that of light, we cannot see stars as they are "now". Even light from the Sun is old. If the Sun exploded right now, we wouldn't know about it for another 8 minutes. However, we think we know how stars evolve and can thus predict what the star would look like at any point in its history.   Top

How is the age of black holes determined ?
(Dimitra Atri)

Black holes forming in our universe occur due to the gravitational collapse of massive stars, (several times the solar mass), at the end point of their evolution when they have exhausted their thermonuclear energy resources. These massive stars are very short-lived as compared to the sun (for instance, a star of about 15 solar masses can last only about ten
million years compared to the five billion odd years of stars like the sun which is indeed fortunate for us !). So black holes in binary systems (which is the only way of surely estimating their mass) can be dated by looking at their companion stars which were presumably formed at the same time. The early type massive stars are usually only several millions of years old, so the black hole ages are comparable. Black holes forming now in our galaxy are likely to be remnants of stars much younger than the sun. However, the supermassive black holes powering the quasars, active galactic nuclei, etc would have formed early on during galactic evolution and are several billion years old. As quasars are at high red shifts they correspond to an earlier epoch of the universe, that is, when the galaxies were in their formative stages and are thus much older than the sun !. Primordial black holes formed in the early universe would have evaporated away by now if their mass is less than about a billion tons. Heavier ones would still be around and would be among the oldest obejcts in the universe ! It must be noted that a solar mass black hole once it forms, would outlive all other celestial obejcts (and all other kinds of matter) as they would take ~ 10 64 years to evaporate !   Top

How does the spectral analysis of stellar objects help ? (Sumit G)

Low resolution spectra are used for spectral classification of objects that is to find the spectral type (an indicator of effective temperature of the star) and luminosity class (an indicator of gravity).

High resolution spectra are used for determination of the effective temperature, gravity, microturbulent velocity of a star. It is also used for determination of elemental abundances which in turn tell us something about the nucleosynthesis reactions that might have taken place and are currently taking place in the object. The high resolution data infact reveal a wealth of information. A complete analysis of the high resolution spectra helps in tracing the evolutionary history and current evolutionary status. It can also be used to infer the wind outflow velocities in objects such as supernovae, planetary nebulae etc.

Could you elaborate on the H-R Diagram ?
(Sumit)

Please visit H-R Diagram which will give you a befitting reply.   Top

Can we determine the absolute magnitude of a star using the spectral type information ? (Sumit)

Yes, we can. Based on the empirical data, tables have been formulated from which the absolute magnitude can be read for a star of a given spectral type. For such a table, refer eg. "Astrophysical Data : Planets and Stars" by K.R. Lang, 1992, Springer-Verlag.   Top

How can the colour index tell the temperature of a star ? (Sumit)

Please visit Colour index of Star for the details.   Top

How do we determine star details in globular clusters ? (Sumit)

Just as one studies field stars, physical parameters for stars in a globular cluster too are determined by CCD imaging and spectroscopic study of the globular cluster members.   Top

Is there a possibility of black hole in the centre of galaxy ?

Infact, recently there has been considerable evidence that the centre of our galaxy harbours a massive black hole weighing a few million solar masses. Indeed several galaxies also show evidence for such massive black holes in their central regions. This is deduced from observing the velocities of the stars (and other objects) within a parsec (that is the innermost region) of the galactic centre. From these velocties, one can deduce the total mass contained in this compact region. It works out to several million solar masses. Only a massive black hole is compact enough to contain so much mass over a such a small length scale.   Top

How do we measure star motion from solar motion ? (Sumit)

The motion of a star can be determined from the radial and tangential velocity measurements done using spectroscopic and different epoch observations respectively (with respect to our line of sight). These velocities essentially have the solar components in it which has to be subtracted. Inorder to avoid sun as the origin of the co-ordinate system, one defines a local standard of rest (LSR) which is an imaginary point near solar neighbourhood going in a circle with respect to galactic centre. The motion of stars including our sun measured with respect to LSR is called peculiar motion. From the systematic observation of stars near solar neighbourhood one can find out the peculiar motion of sun. Once this is known, the observed motion of any star can be transformed to LSR.

Velocity of LSR = 250 Kms-1

Peculiar velocity of sun in space with respect to LSR is

U sun = -9 Kms-1

V sun = 12 Kms-1

W sun = 7 Kms-1

Reference : Mihalas & Binney, Galactic Astronomy (pp 380)   Top

Does the convective equilibrium affect the hydrostatic equilibrium of a star ? (Sumit)

Energy equilibrium set by the energy transport processes such as convection is in general coupled to the mechanical force equilibrium of a star.But once a star achieves a global force and energy equilibrium, the energy transport processes go on in a constant manner so as to maintain the hydrostatic equilibrium of a star. Thus energy equilibrium is a requirement
for the hydrostatic equilibrium to hold and vice versa. Alteration of one will lead to the alteration of the other. The term 'convective equilibrium' is used to describe those layers of a star where all the energy is transported by convective motions and in a constant manner, i.e., the convective energy flux is constant through the layers. Strictly, the divergence of convective flux being zero is referred to as convective equilibrium. Thus, convective equilibrium in the convection zone of a star is coupled to the hydrostatic equilibrium in the sense that violation of one lead to the violation of the other and eventually leading to the restoration of equlibria. But, it is to be noted that though the convective motions lead to loacl deviations from
hydrostatic condition in the sense that the material is not static, globally stationary convection still maintains the hydrostatic equilibrium of a star.

Background Information:

Hydrostatic equilibrium refers to the force balance between the gas pressure and the gravitational force (the weight of the material) and is a consequence of one (momentum) of the three basic conservation laws, viz., the mass momentum and energy conservations. The global equilibrium of a star is enforced by all of these conservation laws and, in general, the physical processes that enforce these requirements are coupled. Heating or cooling of gas at one location inside a star, say by radiation, affects the thermal properties of the gas like the pressure and temperature which in turn affect the momentum balance. Such interactions take place in the initial formative stages of a star, which eventually lead to a equilibrium situation wherein everything is in balance. Any deviation in the energy or momentum equilibrium leads instantly to such interactions which act to restore equilibrium.   Top

Do stars that are near the galactic centre move around it faster or those that are in the far away arms ? (Sumith)

From the motion of stars near the galactic centre, a presence of a massive black hole having a mass of a few million solar masses has been deduced. Knowing the velocities of the stars in the innermost parsec or so, enables us to deduce using Kepler's laws that the mass contained within that small radius is a few million solar masses. Only a black hole
can be so compact. A compact collection of massive stars over such a radius would radiate an enormous amount of energy with distinct spectral characteristics.

If most of the galactic mass were confined to the central regions, the velocity of stars further apart would fall with increasing distance from the centre. However most spiral galaxies including our own show what is called a 'flat rotation curve', that is, the velocity tends to 'level off' beyond some distance. This is evidence for so called 'dark matter' (which does not radiate, there is no light beyond some radius) which supplies thenecessary dynamical mass in the outer regions to keep the velocity from falling.   Top

If we see a blue star surrounded by a red nebula, is this nebula emission or reflection ?

If gas and dust cloud surrounds a star or a group of stars, then the cloud shines by reflected light. The reflection nebula, as they are called, looks blur than the light from the source star. This is because the interstellar dust grains scatter blue light more than red light.

Gas clouds surrounding a hot bright star, or a group of stars, can shine in visual part of the spectrum due to a mechanism called fluroscence. This mechanism works in (a) planetary nebulae, where a recently ejected circumstellar gas shell is lit up by a hot dying star. (b) HII region, where a neighbouring gas cloud is lit up by one or more newborn O or B stars.

These regions are dominated by Balmer line and forbidden line emission. This is the reason why the nebula looks prominent when the image is taken through H-alpha (6563 Angstroms) filter.   Top

What is the reason for the increase in size of a star when it reaches Red Giant Star ?

After a star forms (hydrogen fusion into helium begins and stabilizes in the core of the star), this stage is called the main sequence. Depending on its mass (that sets the rate of burning of hydrogen in the core of the star and the available mass) it remains on the main sequence for about M(-2.5) 1010 years, (you can think about how this timescale is derived) where M is units of the sun's mass. The sun's life on the main sequence is about 10 billion years.

Now again depending on the mass after the hydrogen fuel is exhausted, the less massive ones ( 2 solar mass) undergo convective burning of H in a non-degenerate core and ultimately shell burning begins.

Now the Red Giant phase is the stage of stellar evolution when star's core contracts after H exhaustion in the core and the envelope expands-which can be understood in the following way-

1.the density just outside the degenerate core is lower after the H exhaustion period, so the temperature goes up to maintain the pressure against self-gravity (ideal gas law).

2.therefore the luminosity (radiation power) increases and temperature gradient increases; the envelope then expands to balance the temperature gradient.

3.the Red giant phase ends when the He core ignites (He burns into carbon through the triple alpha reaction[?]).

For further information, please read
Physical Universe by Frank Shu

Astrophysical Concepts by Martin Harwitt

(These books are suitable for physics undergraduates)

What will happen if the core of a main sequence star gets too hot ?

A main sequence star is in hydrostatic equilibrium. The gravitational force due to its mass is balanced by the radiation and thermal pressures that is maintained due to the energy produced by nuclear reactions in the core. If the core of the star gets too hot, it leads to higher rates of nuclear reactions at the center, then the star expands to reduce its temperature. However, a main sequence star burns hydrogen into helium and towards the end of its main sequence life (and depending on its mass), it begins to burn helium (or a hydrogen shell burns around a degenerate hydrogen core). This is called the red giant phase.

Depending on the mass after the hydrogen fuel is exhausted, the less massive ones ( 2 solar mass) undergo convective burning of H in a non-degenerate core and ultimately shell burning begins.

Now the Red Giant phase is the stage of stellar evolution when star's core contracts after H exhaustion in the core and the envelope expands - which can be understood in the following way - 1.the density just outside the degenerate core is lower after the H exhaustion period, so the temperature goes up to maintain the pressure against self-gravity (ideal gas law).
2.therefore the luminosity (radiation power) increases and temperature gradient increases; the envelope then expands to balance the temperature gradient.
3.the Red giant phase ends when the He core ignites (He burns into carbon through the triple alpha reaction[?]).

For further information, please read

Physical Universe by Frank Shu & Astrophysical Concepts by Martin Harwitt.   Top

Last updated on: February 20, 2024