APOD: VAR (2011 Jul 01)

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APOD: VAR (2011 Jul 01)

Post by APOD Robot » Fri Jul 01, 2011 4:09 am

Image VAR

Explanation: In the 1920s, examining photographic plates from the Mt. Wilson Observatory's 100 inch telescope, Edwin Hubble determined the distance to the Andromeda Nebula, decisively demonstrating the existence of other galaxies far beyond the Milky Way. His notations are evident on the historic plate image inset at the lower right, shown in context with ground based and Hubble Space Telescope images of the region made nearly 90 years later. By intercomparing different plates, Hubble searched for novae, stars which underwent a sudden increase in brightness. He found several on this plate and marked them with an "N". Later, discovering that the one near the upper right corner (marked by lines) was actually a type of variable star known as a cepheid, he crossed out the "N" and wrote "VAR!". Thanks to the work of Harvard astronomer Henrietta Leavitt, cepheids, regularly varying pulsating stars, could be used as standard candle distance indicators. Identifying such a star allowed Hubble to show that Andromeda was not a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Hubble's discovery is responsible for establishing our modern concept of a Universe filled with galaxies.

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Re: APOD: VAR (2011 Jul 01)

Post by Beyond » Fri Jul 01, 2011 4:35 am

What a difference a bigger well placed telescope makes.If Hubble could have looked through the telescope we now have in space with his name on it, he would have said WOW, after he got up off the floor AND put his socks back on. :mrgreen:
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Re: APOD: VAR (2011 Jul 01)

Post by neufer » Fri Jul 01, 2011 5:42 am

APOD Robot wrote:Image VAR

Explanation: Thanks to the work of Harvard astronomer Henrietta Leavitt, cepheids, regularly varying pulsating stars, could be used as standard candle distance indicators. Identifying such a star allowed Hubble to show that Andromeda was... a large galaxy in its own right at a substantial distance from the Milky Way. Hubble's discovery is responsible for establishing our modern concept of a Universe filled with galaxies.
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Re: APOD: VAR (2011 Jul 01)

Post by Ann » Fri Jul 01, 2011 6:18 am

Well, it's a good thing Edwin Hubble found his cepheid variable in the Adromeda Galaxy so that he could prove that the Andromeda Galaxy really was the Andromeda Galaxy, after Harlow Shapley had proved, just a few years earlier in the "Great Debate" against Heber Curtis, that the Andromeda Galaxy was just a small gas cloud inside the Milky Way.

http://en.wikipedia.org/wiki/The_Great_Debate writes:
Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed galaxies such as Andromeda and the Spiral Nebulae were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda was not part of the Milky Way, then its distance must have been on the order of 10 to the power of 8 light years—a span most astronomers would not accept. Adriaan van Maanen was also providing evidence to Shapley's argument. Van Maanen was a well-respected astronomer of the time who claimed he had observed the Pinwheel Galaxy rotating. If the Pinwheel Galaxy were in fact a distinct galaxy and could be observed to be rotating on a timescale of years, its orbital velocity would be enormous and there would clearly be a violation of the universal speed limit, the speed of light. Also used to back up his claims was the observation of a nova in the Andromeda galaxy that had temporarily outshone the nucleus of the galaxy itself, a seemingly absurd amount of energy for a normal nova. Thus, the nova and the galaxy itself must be within our own galaxy since, if Andromeda were a galaxy in its own right, the nova would have had to have been unthinkably bright in order to be seen from so far away.
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Why the `Great Debate' Was Important

Post by neufer » Fri Jul 01, 2011 11:48 am

Ann wrote:
http://en.wikipedia.org/wiki/The_Great_Debate wrote:
Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed galaxies such as Andromeda and the Spiral Nebulae were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda was not part of the Milky Way, then its distance must have been on the order of 10 to the power of 8 light years—a span most astronomers would not accept. Adriaan van Maanen was also providing evidence to Shapley's argument. Van Maanen was a well-respected astronomer of the time who claimed he had observed the Pinwheel Galaxy rotating. If the Pinwheel Galaxy were in fact a distinct galaxy and could be observed to be rotating on a timescale of years, its orbital velocity would be enormous and there would clearly be a violation of the universal speed limit, the speed of light. Also used to back up his claims was the observation of a nova in the Andromeda galaxy that had temporarily outshone the nucleus of the galaxy itself, a seemingly absurd amount of energy for a normal nova. Thus, the nova and the galaxy itself must be within our own galaxy since, if Andromeda were a galaxy in its own right, the nova would have had to have been unthinkably bright in order to be seen from so far away.
Well, it's a good thing Edwin Hubble found his Cepheid variable in the Andromeda Galaxy so that he could prove that the Andromeda Galaxy really was the Andromeda Galaxy, after Harlow Shapley had proved, just a few years earlier in the "Great Debate" against Heber Curtis, that the Andromeda Galaxy was just a small gas cloud inside the Milky Way.
At least Shapely got the size of our galaxy and our relative position in it correct.

Curtis couldn't imagine that galaxies were as big as they actually are.

Shapely couldn't imagine that the Universe was as big as it actually is.
http://apod.nasa.gov/htmltest/gifcity/cs_why.html wrote:
<<Although the `Great Debate' is important to different people for different reasons, it is a clear example of humanity once again striving to find its place within the cosmic order. In the debate, Shapley and Curtis truly argued over the ``Scale of the Universe," as the debate's title suggests. Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as "spiral nebulae". Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy.

In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. Although the fine points of the debate were more numerous and more complicated, each scientist disagreed with the other on these crucial points.

A partial resolution of the debate came in the mid-1920's. Using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy (M31) . These stars allowed Hubble to show that the distance to M31 was greater than even Shapley's proposed extent of our Milky Way galaxy. Therefore M31 was a galaxy much like our own. In the 1930s, the further discovery of interstellar absorption combined with an increased understanding of the distances and distribution of globular clusters ultimately led to the acceptance that the size of our Milky Way Galaxy had indeed been seriously underestimated and that the Sun was not close to the center. Therefore, Shapley was proved more correct about the size of our Galaxy and the Sun's location in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that ``spiral nebulae" were indeed galaxies just like our own. >>
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Re: APOD: VAR (2011 Jul 01)

Post by JuanAustin » Fri Jul 01, 2011 11:53 am

Just curious from comparing the old and new photos of andromeda, is the rotation of galaxies around their galactic centers detectable from observing their arms filled with gas and dust and stellar material as a whole or is it too slow to perceive? If it's not possible over the short amount of time quality photographs or other techniques have been available in modern times, how long would it take to actually observe some rotation?
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Re: APOD: VAR (2011 Jul 01)

Post by biddie67 » Fri Jul 01, 2011 1:03 pm

What a fascinating glimpse into the history of the work and disagreements that went into understanding more of the structure and sizes/distances in the universe.

I have no idea of the physics or math behind the concept of how "regularly varying pulsating stars" can reveal distances. So I have no idea how, on a very high, superficial level, distances can be determined from them. Does anyone have any links to a simple explanation for this - maybe something for a school child (which obviously I'm not but also obviously I need!).

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Re: APOD: VAR (2011 Jul 01)

Post by Celestial » Fri Jul 01, 2011 1:57 pm

biddie67, the link below has a short explanation for a general audience; see if in it you can find what you are looking for:

http://science.howstuffworks.com/question224.htm

howstuffworks: "How are astronomers able to measure how far away a star is?"

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Re: APOD: VAR (2011 Jul 01)

Post by Chris Peterson » Fri Jul 01, 2011 2:28 pm

biddie67 wrote:I have no idea of the physics or math behind the concept of how "regularly varying pulsating stars" can reveal distances. So I have no idea how, on a very high, superficial level, distances can be determined from them.
Variable stars in themselves are not useful for determining distance. What is described here is a very specific kind of variable star, called a Cepheid (named for the first one discovered, Delta Cepheus). This is a class of star which has a rate of pulsation proportional to its absolute brightness. Normally, we can't measure absolute brightness, only apparent brightness. But we can accurately measure the pulsation rate. So if we observe a distant Cepheid, we measure the period of its variability, use that to determine the absolute brightness, and then use the inverse square law of intensity to determine what its distance must be to produce the brightness we actually observe.
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Re: APOD: VAR (2011 Jul 01)

Post by Chris Peterson » Fri Jul 01, 2011 2:34 pm

Ann wrote:Well, it's a good thing Edwin Hubble found his cepheid variable in the Adromeda Galaxy so that he could prove that the Andromeda Galaxy really was the Andromeda Galaxy, after Harlow Shapley had proved, just a few years earlier in the "Great Debate" against Heber Curtis, that the Andromeda Galaxy was just a small gas cloud inside the Milky Way.
Ann, I think you know this already, but we should be clear about things: neither man proved anything. Each was successful in presenting evidence that many people found persuasive. Of course, new data was collected over time, and new theories developed to explain that data, so it is only natural that the mainstream beliefs changed as well. That is the beauty of science.

Even now, we really have no proof that Hubble was correct. All we have is very, very compelling evidence.
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Re: APOD: VAR (2011 Jul 01)

Post by biddie67 » Fri Jul 01, 2011 2:35 pm

Thanks Celestial !!

Quoting from the 2nd paragraph found in your link above:
There is no direct method currently available to measure the distance to stars farther than 400 light years from Earth, so astronomers instead use brightness measurements. It turns out that a star's color spectrum is a good indication of its actual brightness. The relationship between color and brightness was proven using the several thousand stars close enough to earth to have their distances measured directly. Astronomers can therefore look at a distant star and determine its color spectrum. From the color, they can determine the star's actual brightness. By knowing the actual brightness and comparing it to the apparent brightness seen from Earth (that is, by looking at how dim the star has become once its light reaches Earth), they can determine the distance to the star.
I can immediately see the complexity in this - with light from the various different type stars with their dominant color spectrums, (assuming all wavelengths besides visible light possibly included in the calculations), being shifted and distorted by all kinds of interstellar matter, my hat's off to the folks even attempting to come up with formulas and instruments to arrive at their estimates/conclusions.

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Re: APOD: VAR (2011 Jul 01)

Post by Dick Henry » Fri Jul 01, 2011 3:26 pm

Beyond wrote:What a difference a bigger well placed telescope makes.If Hubble could have looked through the telescope we now have in space with his name on it, he would have said WOW, after he got up off the floor AND put his socks back on. :mrgreen:
I never met Hubble, but from what I've heard about him, he was a snob and social climber - and he never believed in the expansion of the universe. And now there is a credible claim that Hubble was NOT the discoverer of the expansion of the universe: http://www.nature.com/news/2011/110627/ ... 1.385.html

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Re: APOD: VAR (2011 Jul 01)

Post by Brad Hansen » Fri Jul 01, 2011 3:30 pm

I just want to say "Thank You!" to R. Gendler for creating this photo-montage juxtaposing historical and modern observations and to APOD for publishing it. It's amazing how far science has progressed in such a short time.

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Re: APOD: VAR (2011 Jul 01)

Post by owlice » Fri Jul 01, 2011 3:38 pm

Dick Henry wrote:I never met Hubble, but from what I've heard about him, he was a snob and social climber - and he never believed in the expansion of the universe. And now there is a credible claim that Hubble was NOT the discoverer of the expansion of the universe: http://www.nature.com/news/2011/110627/ ... 1.385.html
We have the explicit proof (a letter) that the English translation of the 1927 Lemaître's paper was done
by ... Lemaître himself, to the request of the Royal Astronomical Society (Prof. Smart ) ! Thus we
can only conclude to an "auto-censorship" ... and to the extraordinary honesty and modesty of the Belgian physicist (which was not the case of Hubble). Now, since we have access to the full Lemaitre archives, we are checking the meaning of a paragraph where Prof. Smart makes a subtle allusion to the part of the 1927 article concerning the "Hubble" relation...
That from the comments to the above-quoted Nature article. They are worth reading.
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Re: APOD: VAR (2011 Jul 01)

Post by biddie67 » Fri Jul 01, 2011 4:45 pm

Sorry mydialup connection seemed to burp....
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Re: APOD: VAR (2011 Jul 01)

Post by biddie67 » Fri Jul 01, 2011 4:57 pm

Chris Peterson wrote: Variable stars in themselves are not useful for determining distance. What is described here is a very specific kind of variable star, called a Cepheid (named for the first one discovered, Delta Cepheus). This is a class of star which has a rate of pulsation proportional to its absolute brightness. Normally, we can't measure absolute brightness, only apparent brightness. But we can accurately measure the pulsation rate. So if we observe a distant Cepheid, we measure the period of its variability, use that to determine the absolute brightness, and then use the inverse square law of intensity to determine what its distance must be to produce the brightness we actually observe.
Thanks Chris for adding your comment - it helped me find the words to explain what isn't clear to me about using a cepheid to determine distance.

I understand being able to determine the rate of variability from minimum to maximum brightness. What I don't understand is there seems to be some kind of assumption that the cepheids all change brightness at the same rate - no matter what their minimums/maximums might appear to be and therefore how long it takes to go from one to the other.

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Re: APOD: VAR (2011 Jul 01)

Post by neufer » Fri Jul 01, 2011 5:44 pm

Image
Georges Lemaître, priest and scientist
Dick Henry wrote:
I never met Hubble, but from what I've heard about him, he was a snob and social climber - and he never believed in the expansion of the universe. And now there is a credible claim that Hubble was NOT the discoverer of the expansion of the universe: http://www.nature.com/news/2011/110627/ ... 1.385.html
Either you're closing your eyes
To a situation you do now wish to acknowledge
Or you are not aware of the caliber of disaster indicated
By the presence of Lemaître in our community.
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I say, Hubble right here in university.
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Re: APOD: VAR (2011 Jul 01)

Post by Ann » Fri Jul 01, 2011 6:15 pm

Biddie67 wrote:
What I don't understand is there seems to be some kind of assumption that the cepheids all change brightness at the same rate - no matter what their minimums/maximums might appear to be and therefore how long it takes to go from one to the other.
No, that's not how it works. What Henrietta Leavitt discovered about the cepheids in the Small Magellanic Cloud was that the brightest cepheids had the longest cycles. The faintest cepheids, on the other hand, changed the fastest. (And if you are wondering how Henrietta Leavitt could tell the brightness changes of stars from stills, much less the period of the change, the answer is of course that she saw many different photos of the same stars and starscapes taken over an extended period. Also, it was her job to classify stars.)

If you observe a cepheid for a sufficiently long time to determine its period, you can tell by its period whether it's a bright or a faint cepheid. Now, thanks to Hubble, astronomers have a good grasp on the absolute brightness of cepheids, not just their relative brightness (relative to each other, that is).

Cepheids have been observed by the Hubble Space Telescope in galaxies in the Virgo Cluster and beyond, and they have been used to determine the distances to individual galaxies.

Ann
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Re: APOD: VAR (2011 Jul 01)

Post by JohnA » Fri Jul 01, 2011 7:26 pm

I'm assuming they took the negative of Hubble's plate for this photo, because isn't most scientific astronomy work done with the photo negative?

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Re: APOD: VAR (2011 Jul 01)

Post by Chris Peterson » Fri Jul 01, 2011 8:03 pm

JohnA wrote:I'm assuming they took the negative of Hubble's plate for this photo, because isn't most scientific astronomy work done with the photo negative?
Not sure which image you are referring to. In today's APOD, all the images are positive.

I wouldn't say that most scientific work is done using negative mappings of images. For automated science, like locating stars, determining their position, and determining their brightness, the raw image data is analyzed. When actual humans are scrutinizing an image for detail, it is common to try many different palettes- gray, primary colors, various pseudocolor orders, heat (a white-orange-black spectrum), and so forth. You would typically try any or all of these in both positive and negative views.

Back in the days of film, the negatives were often used for making primary measurements since the process of producing a contact positive would add some error to the data. It has been quite some time since film was used for scientific astronomical imaging, however.
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Re: APOD: VAR (2011 Jul 01)

Post by neufer » Fri Jul 01, 2011 8:04 pm

JohnA wrote:
I'm assuming they took the negative of Hubble's plate for this photo,
because isn't most scientific astronomy work done with the photo negative?
http://asterisk.apod.com/viewtopic.php?f=29&t=23744
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Re: APOD: VAR (2011 Jul 01)

Post by Chris Peterson » Fri Jul 01, 2011 8:11 pm

Dick Henry wrote:I never met Hubble, but from what I've heard about him, he was a snob and social climber - and he never believed in the expansion of the universe. And now there is a credible claim that Hubble was NOT the discoverer of the expansion of the universe: http://www.nature.com/news/2011/110627/ ... 1.385.html
Well, many discoverers that get credit for something are not the first on the scene, or there are many others who are very close. Newton used his influence to suppress work by Halley; had Newton never been born we'd have had pretty much the same theory at the same time. Likewise for Copernicus and Kepler. And with more modern science, things move even faster and easier. If Einstein didn't exist, we'd still have had General Relativity at about the same time- lots of people were homing in on it. Likewise for quantum mechanics.

New discoveries in science and technology are really not driven by individuals, but by the inertia of the scientific community itself. You'd be hard pressed to come up with many examples in modern science or technology, which are associated with specific people, that wouldn't have happened without them.

Hubble gets the credit here because he did some of the work and had the loudest and most persuasive voice. Often, that's how it works.
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Re: APOD: VAR (2011 Jul 01)

Post by neufer » Fri Jul 01, 2011 8:21 pm

http://www.daviddarling.info/encyclopedia/R/RR_Lyrae_star.html wrote:
<<A short-period, yellow or white giant pulsating variable; RR Lyrae stars belong to Population II and are often found in globular clusters (hence one of their older names – cluster variables) or elsewhere in the galactic halo. They have periods of 0.2 to 2 days, amplitudes of 0.3 to 2 magnitudes, and spectral types of A2 to F6. Some of them have similar light curves to those of Cepheid variables and, like Cepheids, obey a period-luminosity relation that enables them to serve as reliable distance indicators. RR Lyrae variables, however, are older, less massive, and fainter (with luminosities typical around 45 L{sun}) than Cepheids.>>
ImageImage
RR Lyrae stars and the Hertzsprung-Russell diagram
------------------------------------------------------------
http://en.wikipedia.org/wiki/RR_Lyrae_variable wrote:
<<RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances. RR Lyraes are pulsating horizontal branch stars of spectral class A (and rarely F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses.

RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. (The average absolute magnitude of an RR Lyrae is 0.75, only 40 or 50 times brighter than our Sun.) Their period is shorter, typically less than one day, sometimes ranging down to seven hours.

The relationship between pulsation period and absolute magnitude of RR Lyraes makes them good standard candles for relatively near objects, especially within the Milky Way. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars.>>

Image

RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheid variables, which are strongly associated with the galactic plane.

Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way.

From 1915 to the 1930s, the RR Lyraes became more accepted as a distinct class of star from classical Cepheids, on account of their shorter periods, different location within the galaxy, and finally, they are chemically different from classical Cepheids, being mostly metal-poor, Population II stars.

RR Lyraes have proven difficult to observe in external galaxies, because of their intrinsic faintness. (In fact, Walter Baade's failure to find them in the Andromeda galaxy led him to suspect that the galaxy was much farther away than predicted, and to re-consider the calibration of Cepheid variables and to propose stellar populations.) They were finally found in the 1980s by Pritchet & van den Bergh in the halo of the Andromeda galaxy, and more recently in its globular clusters by the Hubble Space Telescope.
Ann wrote:
Wikipedia says this about Walter Baade:
He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the first time, which led him to define distinct "populations" for stars (Population I and Population II). The same observations led him to discover that there are two types of Cepheid variable stars. This discovery led him to recalculate the size of the known universe, doubling the previous calculation made by Hubble in 1929. He announced this finding to considerable astonishment at the 1952 meeting of the International Astronomical Union in Rome.
Polaris is a classic Population I Cepheid variable (although it was once thought to be Population II due to its high galactic latitude). Since Cepheids are an important standard candle for determining distance, Polaris (as the closest such star) is heavily studied. Around 1900, the star luminosity varied ±8% from its average (0.15 magnitudes in total) with a 3.97 day period; however, the amplitude of its variation has been quickly declining since the middle of the 20th century. The variation reached a minimum of 1% in the mid 1990s and has remained at a low level. Over the same period, the star has brightened by 15% (on average), and the period has lengthened by about 8 seconds each year.

http://asterisk.apod.com/viewtopic.php? ... 91#p137482
http://www.skyandtelescope.com/news/15548412.html wrote: Light Echoes Give Accurate Cepheid Distance
RS Puppis and surrounding nebula
http://apod.nasa.gov/apod/ap080212.html

<<For nearly a century now, astronomers have used Cepheid variable stars as "standard candles" whose apparent brightnesses tell how far away they are. Starting in 1912 Cepheids provided the first good distances to nearby galaxies. One of the reasons for building the Hubble Space Telescope was to measure Cepheids in galaxies farther out than could be done through Earth's fuzzy atmosphere. Indeed, "Hubble" was something of a double-entendre. The name honored the late Edwin Hubble, but the telescope was also intended to pin down the Hubble constant — the expansion rate of the universe — by comparing the redshifts of key galaxies to their distances found using Cepheid variables.

However, this only works if you know the distances to local Cepheids in our own galaxy well enough to calibrate the Cepheid distance scale as a whole. They're rather unusual super giant stars, so none of them lie close enough to the solar system for really accurate parallax measurements of their distances. Accordingly, astronomers have been expending great efforts to deduce local Cepheids' distances accurately in any way they can.

The best such measurements recently attained an accuracy of just a few percent. Now a group of astronomers has broken that record — by using a unique method to get a range on the bright Cepheid RS Puppis good to about 1.4 percent. They did it by measuring "light echoes" of the star's pulsations on a surrounding reflection nebula, combined with the star's accurately known pulsation period, the speed of light, and some simple geometry.

RS Puppis varies in brightness (from magnitude 6.5 to 7.6) every 41.4 days. It is 10 times more massive than the Sun, 200 times larger, and on average 15,000 times more luminous. Pierre Kervella and his colleagues used the European Southern Observatory's New Technology Telescope at La Silla, Chile, to record the faint reflections of these light pulses moving across the nebula. The speed at which they appeared to move, combined with the known speed of light, gave the distance to the nebula and star: 6,500 light years plus or minus 90.

RS Pup is the only Cepheid embedded in a large nebula. "Light that travels from the star to a dust grain to the telescope arrives a bit later than light that comes directly from the star to the telescope," explains Kervella. "As a consequence, if we measure the brightness of a particular, isolated dust blob in the nebula, we obtain a brightness curve that has the same shape as the variation of the Cepheid, but shifted in time." The delay is called a "light echo," by analogy with a sound echo off, say, a canyon wall.

"Knowing the distance to a Cepheid star with such an accuracy proves crucial to the calibration of the period-luminosity relation of this class of stars," says Kervella. RS Pup is especially important because it's one of the longest-period nearby Cepheids, and few of these have been well measured. The new result should help firm up the entire cosmic distance scale.>>
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Re: APOD: VAR (2011 Jul 01)

Post by Guest » Fri Jul 01, 2011 9:00 pm

Awesome, awesome, awesome.

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Re: APOD: VAR (2011 Jul 01)

Post by saturn2 » Fri Jul 01, 2011 10:49 pm

Henrietta Leavit studied the cepheids, variable stars. Remember astronomer Leavit in the woman cientific year.