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#1
September 14th 04, 02:23 PM
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anybody know how to work out the mass of a star from it's spectrum? this is for a school project and it's due on Friday! If you are going to respond please, answer soon
Hello, I'm doing a science project at school which involves the question
of how astronomers detect black holes and properties of black holes. Part of this project is to ask a scientist, or universitry student studying science, questions for our project that we don't understand.I would most gratuitous if any scientist, or university science student, answered these questions. Please help me to my project by answering these questions. I have some questions on black hole detection ,via working out mass of other star in binary 'star' pair.So far I have been found out that a star's mass can be figured out by looking at it's spectrum. I have found out that each element leaves a unique color on spectrum, and so by looking at the colors on a spectrum you can work out the elements in a star and ,if you knew how much of it's element the star was composed off, you could therefore work out the mass of the star. But I don't know how to work out the persentage of each element in the star and therefore can't work out it's weight. Here are my questions.. When you try to work out the mass of a star from it's spectrum,by finding the stars element composition ,the different elements and their percentage in the star, and by finding the weight of these elements), how do you work out what percentage each element is in the star? Do you just base your results ,on the percentage of each element, by how 'thick' each line appears on the spectrum compared to the others, by doing an overall scale of the spectrum and determining it's ratio compared to other elements? Can you please tell me more information about this process? How do you account for dust clouds ,if any, between the star and the Earth that might affect the spectrum of the star? How do you factor in the hubble constant to prevent a gravitational red-shift in the stars spectrum? When you do get a black hole mass in the end, how do you factor in the changing mass due to Hawking radiation? Why is the gravitational slope of a massive black hole more gentle ,lower gradient, than a smaller black hole, which is stepper? What's upper mass limit for neutron stars and pulsars? Thank you for taking the time to read these questions. Please, I beg you, please consider answering these questions. Your time is much appreciated.Also, if you have any other information on how to calcualte a star's mass from it's spectrum it would be much appreciated. Thank you..... Christine Mc Meikan.... 
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#2
September 14th 04, 03:29 PM
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It can't be done just from its spectrum. You also need its distance and
diameter. It also helps if there are detected planets orbiting it. Once you have the diameter you can calculate the volume. Once you have the volume then you can use the spectrum to determine where the star is on its life cycle and what it is currently using as fuel. With all of these factors you should be able to come up with a good approximation of the stellar mass. Andrew McMeikan wrote: Hello, I'm doing a science project at school which involves the question of how astronomers detect black holes and properties of black holes. Part of this project is to ask a scientist, or universitry student studying science, questions for our project that we don't understand.I would most gratuitous if any scientist, or university science student, answered these questions. Please help me to my project by answering these questions. I have some questions on black hole detection ,via working out mass of other star in binary 'star' pair.So far I have been found out that a star's mass can be figured out by looking at it's spectrum. I have found out that each element leaves a unique color on spectrum, and so by looking at the colors on a spectrum you can work out the elements in a star and ,if you knew how much of it's element the star was composed off, you could therefore work out the mass of the star. But I don't know how to work out the persentage of each element in the star and therefore can't work out it's weight. Here are my questions.. When you try to work out the mass of a star from it's spectrum,by finding the stars element composition ,the different elements and their percentage in the star, and by finding the weight of these elements), how do you work out what percentage each element is in the star? Do you just base your results ,on the percentage of each element, by how 'thick' each line appears on the spectrum compared to the others, by doing an overall scale of the spectrum and determining it's ratio compared to other elements? Can you please tell me more information about this process? How do you account for dust clouds ,if any, between the star and the Earth that might affect the spectrum of the star? How do you factor in the hubble constant to prevent a gravitational red-shift in the stars spectrum? When you do get a black hole mass in the end, how do you factor in the changing mass due to Hawking radiation? Why is the gravitational slope of a massive black hole more gentle ,lower gradient, than a smaller black hole, which is stepper? What's upper mass limit for neutron stars and pulsars? Thank you for taking the time to read these questions. Please, I beg you, please consider answering these questions. Your time is much appreciated.Also, if you have any other information on how to calcualte a star's mass from it's spectrum it would be much appreciated. Thank you..... Christine Mc Meikan....
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#4
September 15th 04, 05:07 AM
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Andrew McMeikan wrote:
Hello, I'm doing a science project at school which involves the question of how astronomers detect black holes and properties of black holes. Part of this project is to ask a scientist, or universitry student studying science, questions for our project that we don't understand.I would most gratuitous if any scientist, or university science student, answered these questions. Please help me to my project by answering these questions. I'm not sure I qualify for your project's requirement, but I *was* "a university student studying science" (a long time ago), and I've done some spectrography (although in analytical chemistry rather than astronomy). [snip] When you try to work out the mass of a star from it's spectrum,by finding the stars element composition ,the different elements and their percentage in the star, and by finding the weight of these elements), how do you work out what percentage each element is in the star? Do you just base your results ,on the percentage of each element, by how 'thick' each line appears on the spectrum compared to the others, by doing an overall scale of the spectrum and determining it's ratio compared to other elements? Can you please tell me more information about this process? You've got the idea, but bear in mind that the spectrum mainly tells us about the composition of the outer layers of a star. We don't see into the interior, because radiation coming from the core is absorbed and re-emitted over and over again as it works its way outward; only where the star becomes thin enough to be transparent can radiation escape into space and be detected. The elemental 'signatures' in a spectrum come in two forms: emission and absorption bands. The former are bright lines produced when an excited atom 'relaxes', releasing a specific amount of energy by emitting a photon of the corresponding wavelength. Material surrounding a star, or in a cooler layer of the outer atmosphere, can absorb thermal radiation whose frequency 'resonates' with certain atoms, so these elements produce dark bands in a spectrum. The relative intensity or density of the lines is a function not only of the source's elemental composition, but also of the prevailing temperature and pressure, and likewise any other conditions that may affect the atoms' behaviour. For example there's a species of oxygen ion found in many nebulae that can only exist at extremely high temperatures and low pressures; when lines from its characteristic electron transitions were first observed in spectra it was thought to be a new element (called "nebulium"), because it had never been seen in a laboratory on earth. How do you account for dust clouds ,if any, between the star and the Earth that might affect the spectrum of the star? See above. The nature of the dust can be determined from the frequencies it absorbs, and if it's comparatively near to us it's likely to have a more or less consistent effect on the spectra of all the stars in that area of the sky. How do you factor in the hubble constant to prevent a gravitational red-shift in the stars spectrum? Very few extragalactic stars are bright enough that their spectra can be picked out from those of their neighbours at 'cosmological' distances, so the Hubble constant will be irrelevant to most observations of this kind. And I don't believe ordinary stars are anywhere near massive enough to produce a detectable gravitational red-shift (which is quite different from the "cosmological red-shift" attributed to the expansion of space). However, most of the stars in our Galaxy are either approaching us or receding fast enough to produce a very noticeable blue- or red-shift, respectively. Whatever shift may occur has no effect on identifying the spectral bands, though, because they all move the same amount, preserving their characteristic patterns. In fact it's exactly because the series of lines remain intact that we know there's a red- or blue-shift in the first place, and the radial component of a star's velocity relative to the Solar System can be determined fairly precisely by comparing the 'standard' frequencies to those observed. In addition to outright shifts, spectral lines may also be subject to "spreading". This is seen from rapidly rotating objects, where light coming from the approaching 'limb' is blue-shifted, and from the receding red-shifted; since we see stars as point sources, we get a mixed signal in which each line appears broadened or 'smeared out'. When you do get a black hole mass in the end, how do you factor in the changing mass due to Hawking radiation? I believe the loss of mass due to Hawking radiation is too slow to make an appreciable difference over any relevant time-scale. Why is the gravitational slope of a massive black hole more gentle ,lower gradient, than a smaller black hole, which is stepper? I'm far from up to speed on the theory of black holes, but it must have to do with their physical dimensions: the inverse-square law of gravitation implies that a smaller radius will trump a proportionally greater mass. What's upper mass limit for neutron stars and pulsars? About 2.3 solar masses, say 4.6*10^27 tonnes. Thank you for taking the time to read these questions. Please, I beg you, please consider answering these questions. Your time is much appreciated.Also, if you have any other information on how to calcualte a star's mass from it's spectrum it would be much appreciated. No need to beg! Saul has alluded to a very important point, that determining the composition of the visible layers of a star gives only 'circumstantial evidence' of its mass: it's not like a spectrographic assay conducted in a lab. Together with overall temperature and luminosity measurements, these clues about elemental composition contribute to models of stellar dynamics, and aid in the classification and identification of stars by size, age, and 'life-history', but they certainly don't provide a direct mass read-out. -- Odysseus
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#5
September 15th 04, 08:51 PM
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A bit more on interstellar dust (and gas): There are also
interstellar spectral lines from dust, but mostly gas. With high enough resolution you can pick these out. The clouds containing this dust and gas moves differently than the star seen through it which separates those lines from the star's. Astronomers have known this for a long time. The term spreading you used is called rotational broadening by astronomers. Saul Levy On Wed, 15 Sep 2004 04:07:02 GMT, Odysseus wrote: Andrew McMeikan wrote: How do you account for dust clouds ,if any, between the star and the Earth that might affect the spectrum of the star? See above. The nature of the dust can be determined from the frequencies it absorbs, and if it's comparatively near to us it's likely to have a more or less consistent effect on the spectra of all the stars in that area of the sky. How do you factor in the hubble constant to prevent a gravitational red-shift in the stars spectrum? In addition to outright shifts, spectral lines may also be subject to "spreading". This is seen from rapidly rotating objects, where light coming from the approaching 'limb' is blue-shifted, and from the receding red-shifted; since we see stars as point sources, we get a mixed signal in which each line appears broadened or 'smeared out'. Saul has alluded to a very important point, that determining the composition of the visible layers of a star gives only 'circumstantial evidence' of its mass: it's not like a spectrographic assay conducted in a lab. Together with overall temperature and luminosity measurements, these clues about elemental composition contribute to models of stellar dynamics, and aid in the classification and identification of stars by size, age, and 'life-history', but they certainly don't provide a direct mass read-out.
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#6
September 17th 04, 03:51 AM
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Saul
Maybe he means that the red/blue shift of the analysis can be used to determine the period. From the period, the distance etc. Am I on the wrong track? BP "Saul Levy" wrote in message ... A bit more on interstellar dust (and gas): There are also interstellar spectral lines from dust, but mostly gas. With high enough resolution you can pick these out. The clouds containing this dust and gas moves differently than the star seen through it which separates those lines from the star's. Astronomers have known this for a long time. The term spreading you used is called rotational broadening by astronomers. Saul Levy
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#7
September 17th 04, 09:32 PM
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I may not understand exactly what you're asking, but...
Yes, periods can be determined by analyzing the shifted lines. Interstellar lines don't shift, by the way. To determine the distance takes more information. You need an orbital solution or the inclination of that orbit and something to give you the scale of the orbit. The radial velocities of the star(s) give one component (line-of-sight velocity). Is that enough for you? Saul Levy On Thu, 16 Sep 2004 19:51:12 -0700, "BP" wrote: Saul Maybe he means that the red/blue shift of the analysis can be used to determine the period. From the period, the distance etc. Am I on the wrong track? BP "Saul Levy" wrote in message .. . A bit more on interstellar dust (and gas): There are also interstellar spectral lines from dust, but mostly gas. With high enough resolution you can pick these out. The clouds containing this dust and gas moves differently than the star seen through it which separates those lines from the star's. Astronomers have known this for a long time. The term spreading you used is called rotational broadening by astronomers. Saul Levy
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