How do we know which stars are close and which stars are far away?
"That's been the single greatest frustration in all of astronomy. Looking at the night sky, even with telescopes, you cannot tell distances. That's been the Holy Grail of astronomy for centuries." -- Michio Kaku, physicist, February 19th 2008
"One of the frightening things I think for conventional astronomers is to accept the fact or to realize that these intrinsic redshifts of the quasars and peculiar galaxies and so forth, active galaxies, means that a lot of the things which we thought were out at great distance in the universe are very much closer in. And in fact you would have to say that what we call the Local Supercluster is much more crowded and contains many more objects than we previously thought." -- Halton C. Arp, astronomer, 2000
Arp, H.C., Cepheid Variables in the Small Magellanic Cloud, Astronomical Journal, Volume 63, Number 2, Page 45, Feb 1958
This difference coupled with the striking difference in amplitude of variation of the shorter period Cepheids opens the question of whether, at a given period, the Cepheids in the galaxy and the SMC have the same luminosity, or mass, or chemical composition.Dear Dr. Arp,
How does one know, by looking at it, if a star is a red giant or a red dwarf?
How do we know the distance to Delta Cephei?
It is said that Delta Cephei is a "yellow-white class F (F5) supergiant" but how do we know it's a supergiant?
Supposing there are frozen sodium ions in between Delta Cephei and the Earth, wouldn't that slow the light down on it's way to the Earth?
Keep up the good work.
Warmest possible regards,
Me
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22 comments:
Dear Oils,
The sodium in those experiments had two very important qualities:
.:They were magnetically confined so as to be very densely packed in a Bose-Einstein Condensate.
.:They were at nanokelvin temperatures.
The coldest natural temperature ever recorded is 1K. 450nK is many orders of magnitude colder than that.
Unless you can show naturally occurring BECs scattered across the universe, I'm afraid you're wrong about your intervening cold gas slowing light down.
However, keep looking! If you find those, you'll be... I don't know what you'll be... but it will be important...you know I want you to be right about something...someday.
Laser-cooled light trapping best wishes,
Jeffery
...but how do we know it's a supergiant?
you can tell the TEMPERATURE of a glowing surface by its COLOR (how red versus how blue) so they measure the surface temp of stars essentially by putting thru a prism and separating out the colors and studying the spectrum (rainbow)
a certain LAW (which can be tested in lab) tells the WATTS OF POWER PER SQUARE METER output of a glowing surface just from the temperature. If you have a square meter of hot iron or ceramic and you tell me the temp, then I can predict the watts of light coming off fairly accurately. It is a beautiful law going back to 1870-1880 if I remember right, one of the most beautiful in all physics. A theorist name Boltzmann and an experimentalist Stefan discovered it. The surface does not have to be solid. It can be the surface of a star, that works too.
with a telescope and light-meter you can TELL THE WATTAGE OF A STAR
If you know the total watts output of a star and you also know the watts per square meter of surface area then you can easily calculate the total surface area.
If you know the surface area you can quickly find the radius because the surface area of a sphere of radius R is 4 pi R2
So finding the radius of a star from what you can tell about it with a telescope comes down mostly to High School Algebra (and a couple of simple laws that can be verified in the laboratory)
--Attributed to Marcus, Physics Forums, Dec 27, 2007
Jeffery,
"you can tell the TEMPERATURE of a glowing surface by its COLOR (how red versus how blue)"
So if a quasar is red that means it has a low temperature and if a quasar is blue it's a high temperature? What is the temperature of a red quasar and what is the temperature of a blue quasar?
"If you have a square meter of hot iron"
This reasoning is circular because it assumes you already know what you are trying to find.
OIM,
Read up blackbody radiation and physics thereof.
Physics is experimental and it establishes yardsticks on measurements and all yardsticks are contantly checked to be valid.
There is no "belief" in old farts, whimps, hot rock throwers, or beamers messing with your brain.
Whatever happened to your decision to go back to school? Do you have to start at first grade?! Have a happy new year!
Spectroscopy of stars reveals enormous details about the chemical composition, pressure, temperature, magnetic field, rotational speed and line-of-sight motion of the star with respect to the Earth. From these things many other facts can be deduced. Imagine looking at pictures of a thousand people of all ages from infancy to old age. And imagine further that you know that every single picture was taken within a few minutes of every other. Could you deduce the fact that people age from infancy to maturity to death? I think you could. Similarly, even though the enterprise of astronomy is young, and all our spectroscopic "pictures" of stars have been collected over an interval short compared to the life spans of stars, we can nevertheless figure out what characteristics indicate age, youth, vast or small size, high or low surface temperature, high or low surface magnetic fields, profligate mass loss or it's absence, and many other things.
Here is a link to a great book which is not only fun to read, but instructive about the ways we learn about stars:
http://www.amazon.com/Hundred-Greatest-Stars-James-Kaler/dp/0387954368
KV,
"Physics is experimental"
What experiment have you performed to determine the distances of stars?
Daitreme,
"Spectroscopy of stars reveals enormous details about the chemical composition"
Aren't all stars made up of hydrogen and helium in the standard model?
"pressure, temperature, magnetic field, rotational speed and line-of-sight motion of the star with respect to the Earth."
So they claim.
Triangulation
OIM,
You asked: What experiment have you performed to determine the distances of stars?
I look up on a night sky, see them far away!
And, I also don't see any beamers going about near or far, and I wonder about the wonderful universe that I can see!
Trigonometry allows direct measurements of stars out to about 1000 light years. Within that range are enough stars of all types to calibrate other distance measuring methods. There is thus a "ladder" of methods used to measure cosmic distances. If you seriously doubt any part of that ladder, there are tons of opportunities to question and test it.
I think the present understanding of that ladder of distance scales is robust and correct within the local group of galaxies and begins to have opportunity for error beyond there.
QF,
The parallax of the nearest star is only .7 seconds of arc LOL.
KV,
My advice to you: go back to preschool.
Diatreme,
I'm going to pick up that book you recommended. Trying to figure this out. Until I figure it out for myself I'm not going to understand this since it's all very confusing.
OIM,
Thank the whimps! I don't follow your advice!
Looks like you heard from the school: they want yo to start at kiddie classes!
WHy don't you go out at night, and look at the stars and see how long before your brain (if there is anything left after beamers have been through) starts classifying them as near, far, something close by and who the heck cares!
Jeffery,
You're right about the sodium because the CMB radiation is 3 degrees Kelvin.
Stars are indeed mostly Hydrogen and Helium, but thanks to spectroscopy, dozens of other elements reveal themselves. All stars massive enough to actually have fusion reactions at their cores convert Hydrogen into Helium and traces of other elements. The more massive stars are, the greater the pressures at their cores, and these higher pressures permit the stars to burn the "ash" left over from the "main sequence" stage when the star is burning only Hydrogen. There are two rather wonderful "miracles" about stars. The first is that they exist at all. The second is that some stars are so massive that they can start start with the lightest elements and burn the ashes leftover from each stage of fusion until the core begins to resemble an onion-skin of layers of elements with higher and higher mass numbers. Then, as the star gets to elements as heavy as Iron, the act of fusing those elements becomes endothermic and the core implodes. This is bad news for the core, and very good news for us. These massive stars create the bottom half of the period table* while they live, and during the hours in which it takes for the star to die in a Supernovae explosion, the explosion creates the top half of the periodic table. Then these by-products of thermonuclear fusion reactions in the stars enrich the interstellar medium so the next generation of stars may have planets like ours. The miracle of these heavy stars is twofold, cooking up the heavier elements and then exploding rather than collapsing when the core collapses.
*There is a measure of the stability of the nuclei of all elements in periodic table. That measure is called the binding-energy-per-nucleon. The Iron nucleus has the maximum value and as a result, all elements lighter than iron release energy when combined in thermo-nuclear fusion reactions, and all elements above Iron release energy in fission reactions. During the final hours in the life of a super-heavy star (say, 60 times the mass of the sun) both processes happen with such vigor that every known element is produced and then blasted back into space. If the Universe were exactly that same as our present one except for the fact of Supernovae explosions, we would not be here, for almost every atom in our bodies heavier than Helium was created in a massive star and dispersed when that star exploded at the end of it's life.
Anyway, getting back to spectra, here is an image of the so-called Fraunhofer lines that are in the spectrum of our nearest star:
http://www.noao.edu/image_gallery/html/im0588.html
Here is a pretty amazing list of databases of astronomical spectra:
http://www.ucm.es/info/Astrof/invest/actividad/spectra.html
This database conveys some idea of how busy astronomers have been over the last hundred years.
Diatreme,
A nice concise summary and great links too. A great way to start the new year, until OIM goes posting some nonsense.
All,Please VOTE on:
Should OIM go back to school?
My vote is: YES, andstudy computational physics.
JK,
OIM has expressed an interest in reading about the subject. This is the most civilized reaction possible.
By the way, here is one of my all-time favorite books:
The Accidental Universe by Paul Davies
http://www.amazon.com/gp/product/0521286921/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0521242126&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0ZNQFN36RXRWQPTV94X9
You can get this book used for about $6.00. You can read it slowly and carefully in about three hours. The Universe then gives you to equally profound alternative scenarios to ponder. The Universe has accidentally arisen and produced sentient beings. Or, the Universe was designed to be inhabited. This books gets you to a location where you can view the subject matter and ponder it with your feet on a solid foundation of physics.
--fred
Diatreme,
"Stars are indeed mostly Hydrogen and Helium"
According to the standard model which is most likely wrong.
"but thanks to spectroscopy, dozens of other elements reveal themselves."
I follow you.
"All stars massive enough to actually have fusion reactions at their cores convert Hydrogen into Helium and traces of other elements."
According to the standard model.
"The more massive stars are, the greater the pressures at their cores"
How do you measure the pressure of the core of a star? How do you even measure the pressure of the core of the Sun?
"and these higher pressures permit the stars to burn the "ash" left over from the "main sequence" stage when the star is burning only Hydrogen."
According to the standard model.
"There are two rather wonderful 'miracles' about stars. The first is that they exist at all. The second is that some stars are so massive that they can start start with the lightest elements and burn the ashes leftover from each stage of fusion until the core begins to resemble an onion-skin of layers of elements with higher and higher mass numbers. Then, as the star gets to elements as heavy as Iron, the act of fusing those elements becomes endothermic and the core implodes. This is bad news for the core, and very good news for us. These massive stars create the bottom half of the period table* while they live, and during the hours in which it takes for the star to die in a Supernovae explosion, the explosion creates the top half of the periodic table. Then these by-products of thermonuclear fusion reactions in the stars enrich the interstellar medium so the next generation of stars may have planets like ours. The miracle of these heavy stars is twofold, cooking up the heavier elements and then exploding rather than collapsing when the core collapses.
*There is a measure of the stability of the nuclei of all elements in periodic table. That measure is called the binding-energy-per-nucleon. The Iron nucleus has the maximum value and as a result, all elements lighter than iron release energy when combined in thermo-nuclear fusion reactions, and all elements above Iron release energy in fission reactions. During the final hours in the life of a super-heavy star (say, 60 times the mass of the sun) both processes happen with such vigor that every known element is produced and then blasted back into space. If the Universe were exactly that same as our present one except for the fact of Supernovae explosions, we would not be here, for almost every atom in our bodies heavier than Helium was created in a massive star and dispersed when that star exploded at the end of it's life."
According to the standard model.
"Anyway, getting back to spectra, here is an image of the so-called Fraunhofer lines that are in the spectrum of our nearest star:
http://www.noao.edu/image_gallery/html/im0588.html
Here is a pretty amazing list of databases of astronomical spectra:
http://www.ucm.es/info/Astrof/invest/actividad/spectra.html
This database conveys some idea of how busy astronomers have been over the last hundred years."
I'm not confused about the spectra. The spectra is the only part of this I understand.
Diatreme,
No such thing as an accident according to the philosophers.
"Some people even question whether they [chance and spontaneity] are real or not. They say that nothing happens by chance, but that everything which we ascribe to chance or spontaneity has some definite cause ...." --Aristotle, Physics, Book II, 350 B.C.
"... if chance were real, it would seem strange indeed, and the question might be raised, why on earth none of the wise men of old in speaking of the causes of generation and decay took account of chance; whence it would seem that they too did not believe that anything is by chance." -- Aristotle, Physics, Book II, 350 B.C.
"Certainly the early physicists found no place for chance among the causes which they recognized...." -- Aristotle, Physics, Book II, 350 B.C.
How do you even measure the pressure of the core of the Sun?
http://homepages.wmich.edu/~korista/starstruct.html
http://imagine.gsfc.nasa.gov/docs/science/know_l2/stars.html
But there is way more than this. Theory can be checked against reality when super massive stars explode. SN 1987A gave us a chance to see the debris thrown out when the star exploded and it confirmed theoretical models of the interior and the nucleosynthesis that went on before the explosion. The mixtures and abundances of isotopes produced in the core and released by the explosion are heavily influenced by the conditions inside the stars before the explosion. Vast amounts of theory were confirmed by observations of this event.
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``Clayton, Colgate, & Fishman: GAMMA-RAY LINES FROM YOUNG SUPERNOVA REMNANTS (1969)'' - Jonathan Jenkins
Abstract:
Clayton, Colgate, and Fishman (1969) predicted observable gamma-ray lines from the decay of Ni-56 and Co-56 in young supernova remnants. I will discuss how this prediction grew out of the pioneering theoretical work of Burbidge, Fowler, and Hoyle in understanding nucleosynthesis, and how the prediction was eventually realized in observations of SN 1987A. One of the driving goals of the Compton Gamma-Ray Observatory was the detection of gamma-ray lines from SNRs, and in the next few years the INTEGRAL mission will follow up with significantly better energy resolution. There are a number of experiments under development (two based at the CfA) which will continue to improve sensitivity and resolution over the next decade, with the twin goals of finding new supernova remnants and better understanding where and when nucleosynthesis occurs in supernovae.
Reading material:
D. D. Clayton, S. A. Colgate, & G. J. Fishman "Gamma-Ray Lines from Young Supernova Remnants" (1969, ApJ, 155, 75)
http://www.cfa.harvard.edu/~kstanek/astro200/Fall2002.html
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Other cool documents concerning SN1987A
http://sn1987a-20th.physics.uci.edu/1415-Beacom.pdf
http://workshop2007.iasf-roma.inaf.it/talks/saturday_morning_dar.pdf
http://prola.aps.org/abstract/RMP/v60/i4/p859_1
http://www.springerlink.com/content/m143761481228191/fulltext.pdf?page=1
Forgive the digression but here is some essential background regarding the debate that went on concerning where and when the "heavy" elements were made:
http://www.aip.org/history/cosmology/ideas/new-genesis.htm
According to the standard model of Mars, this should be impossible:
http://video.filestube.com/video,f304a7b72d98dcc203e9.html
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