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Thread: Keep Looking Up!

  1. #166
    Senior Member kv466's Avatar
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    I appreciate that, Luna...especially coming from you.
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  2. #167
    Senior Member Lunasong's Avatar
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    Something in kv466's narration intrigued me - that there are green stars. We don't see them as green. Why not? I found this blogpost that explains why.

    We donít see any green stars at all. Hereís why.
    Take a blowtorch (figuratively!) and heat up an iron bar. After a moment it will glow red, then orange, then bluish-white. Then itíll melt. Better use a pot holder.
    Why does it glow? Any matter above the temperature of absolute zero (about -273 Celsius) will emit light. The amount of light it gives off, and more importantly the wavelength of that light, depends on the temperature. The warmer the object, the shorter the wavelength.
    Cold objects emit radio waves. Extremely hot objects emit ultraviolet light, or X-rays. At a very narrow of temperatures, hot objects will emit visible light (wavelengths from roughly 300 nanometers to about 700 nm).
    Mind you ó and this is critical in a minute ó the objects donít emit a single wavelength of light. Instead, they emit photons in a range of wavelengths. If you were to use some sort of detector that is sensitive to the wavelengths of light emitted by an object, and then plotted the number of them versus wavelength, you get a lopsided plot called a blackbody curve. Itís a bit like a bell curve, but it cuts off sharply at shorter wavelengths, and tails off at longer ones.
    Hereís an example of several curves, corresponding to various temperatures of objects.

    The x-axis is wavelength, and the spectrum of visible colors is superposed for reference. You can see the characteristic shape of the blackbody curve. As the object gets hotter, the peak shifts to the left, to shorter wavelengths.
    An object that is at 4500 Kelvins (about 4200 Celsius or 7600 F) peaks in the orange part of the spectrum. Warm it up to 6000 Kelvin (about the temperature of the Sun, 5700 C or 10,000 F) and it peaks in the blue-green. Heat it up more, and the peaks moves into the blue, or even toward shorter wavelengths. In fact, the hottest stars put out most of their light in the ultraviolet, at shorter wavelengths than we can see with our eyes.
    Now wait a sec (again)Ö if the Sun peaks in the blue-green, why doesnít it look blue-green?
    Ah, this is the key question! Itís because it might peak in the blue-green, but it still emits light at other colors.
    Look at the graph for an object as hot as the Sun. That curve peaks at blue-green, so it emits most of its photons there. But it still emits some that are bluer, and some that are redder. When we look at the Sun, we see all these colors blended together. Our eyes mix them up to produce one color: white. Yes, white. Some people say the Sun is yellow, but if it were really yellow to our eyes, then clouds would look yellow, and snow would too (all of it, not just some of it in your back yard where your dog hangs out).
    OK, so the Sun doesnít look green. But can we fiddle with the temperature to get a green star? Maybe one thatís slightly warmer or cooler than the Sun?
    It turns out that no, you canít. A warmer star will put out more blue, and a cooler one more red, but no matter what, our eyes just wonít see that as green.
    The fault lies not in the stars (well, not entirely), but within ourselves.
    Our eyes have light-sensitive cells in them called rods and cones. Rods are basically the brightness detectors, and are blind to color. Cones see color, and there are three kinds: ones sensitive to red, others to blue, and the third to green. When light hits them, each gets triggered by a different amount; red light (say, from a strawberry) really gets the red cones juiced, but the blue and green cones are rather blasť about it.
    Most objects donít emit (or reflect) one color, so the cones are triggered by varying amounts. An orange, for example, gets the red cones going about twice as much as the green ones, but leaves the blue ones alone. When the brain receives the signal from the three cones, it says "This must be an object that is orange." If the green cones are seeing just as much light as the red, with the blue ones not seeing anything, we interpret that as yellow. And so on.
    So the only way to see a star as being green is for it to be only emitting green light. But as you can see from the graph above, thatís pretty much impossible. Any star emitting mostly green will be putting out lots of red and blue as well, making the star look white. Changing the starís temperature will make it look orange, or yellow, or red, or blue, but you just canít get green. Our eyes simply wonít see it that way.
    Thatís why there are no green stars. The colors emitted by stars together with how our eyes see those colors pretty much guarantees it.
    But that doesnít bug me. If youíve ever put your eye to a telescope and seen gleaming Vega or ruddy Antares or the deeply orange Arcturus, you wonít mind much either. Stars donít come in all colors, but they come in enough colors, and theyíre fantastically beautiful because of it.
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  3. #168
    Senior Member Lunasong's Avatar
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    Transited Venus and her regal partner Jupiter are now creating a spectacular morning show similar to the one we enjoyed in the evening earlier this year. In the Northern Hemisphere, look ENE before dawn to see these two bright heavenly objects.
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  4. #169
    Senior Member Lunasong's Avatar
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    Taken by Mark E. White, posted on EarthSky Tonight.

    You can also see the Pleiades in this picture, the little group of stars @ center top.
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  5. #170
    Senior Member Lunasong's Avatar
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    This morning, Venus was shining incredibly bright. It is currently at its brightest and Jupiter is just past its brightest. Wake up early and see this show in the eastern sky before sunrise! For the next couple days, the moon will be near its new phase which makes for a very dark backdrop to enjoy the third and fourth brightest heavenly bodies.
    "To be a musician is a curse. To NOT be one is even worse." Jack Daney

  6. #171
    Senior Member kv466's Avatar
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    Quote Originally Posted by Lunasong View Post
    Transited Venus and her regal partner Jupiter are now creating a spectacular morning show similar to the one we enjoyed in the evening earlier this year. In the Northern Hemisphere, look ENE before dawn to see these two bright heavenly objects.

    I saw a lovely triangle the other morning! I haven't been around here if not I would have posted...slowly making my way back around and getting more time on my hands. Glad you've been watching!
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  7. #172
    Senior Member Lunasong's Avatar
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    The Perseid meteor shower should be at its best late tonight. Find a dark spot with a wide-open view of the sky overhead, bundle up against the late-night chill, lie back in a lounge chair, watch the sky, and be patient. After 11 or midnight you may see a meteor a minute on average; fewer earlier.




    The meteors can flash into view anywhere in the sky as long as Perseus is above the horizon. So the best part of the sky to watch is wherever is darkest, probably straight up.
    Last edited by Lunasong; Aug-12-2012 at 00:23.
    "To be a musician is a curse. To NOT be one is even worse." Jack Daney

  8. #173
    Senior Member Lunasong's Avatar
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    Venus and Jupiter are still the brightest objects in the eastern Northern Hemisphere pre-dawn sky. This morning, I say Jupiter nicely paired up with red star Aldebaran. The constellation Orion lies in-between Jupiter and Venus, just a bit to the right. Dog Star Sirius is now above the horizon.

    If you are now waking up for early morning classes, take the time to look for Venus and Jupiter. They are visible until the sun just breaks above the horizon.
    Last edited by Lunasong; Aug-23-2012 at 14:15.
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    "To be a musician is a curse. To NOT be one is even worse." Jack Daney

  9. #174
    Senior Member Kopachris's Avatar
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    To add to Lunasong's most recent post, the eastern sky at dawn is definitely where the action is right now. Depending on your viewing conditions, you may be able to see the entire winter hexagon just before sunrise, and perhaps the planet Mercury, as well.

    For those interested in astrology (for whatever reason), the Sun is in Leo, Mercury is in Cancer, Venus is in Gemini, and Jupiter is in Taurus, all lined up. Whatever that's supposed to mean.
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  10. #175
    Senior Member Kopachris's Avatar
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    Quote Originally Posted by Kopachris View Post
    For those interested in astrology (for whatever reason), the Sun is in Leo, Mercury is in Cancer, Venus is in Gemini, and Jupiter is in Taurus, all lined up. Whatever that's supposed to mean.
    I don't put any faith in astrology, but I did some research and found the following: Mercury in Cancer has a trine with Jupiter, a square with the Moon, and a semisextile with Venus. For Cancers such as myself, this should mean this is a good time for education and communication, though overcoming emotions and urges may present a challenge, and a conscious effort may have to be put toward beauty and love. This is an interesting coincidence in that I started to become more interested in my education about the time Mercury entered Cancer. I just spent two hours researching astrology to come to the conclusion that now is a good time for education. o.O

    Of course, I still don't know much about astrology, so my interpretation is probably way off of what a more knowledgeable astrologer would have.
    Last edited by Kopachris; Aug-23-2012 at 09:23.
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  11. #176
    Senior Member Lunasong's Avatar
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    If I were on Mars, this would be naked-eye astronomy. I love this pic.
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    "To be a musician is a curse. To NOT be one is even worse." Jack Daney

  12. #177
    Senior Member Meaghan's Avatar
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    Hi folks!
    I'm looking at this thread because I just started taking an astronomy course at my college. We have a little planetarium and we all sat in it this evening while the professor showed us some constellations and how the sun's arc across the sky changes with the seasons and why Mercury appears to move back and forth and some coordinate systems for locating things in the sky and some other stuff. I definitely didn't take it all in, but it's exciting! So I'm just poking my head in to say that I'm looking forward to gaining some knowledge of astronomy and getting involved with this thread!
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  13. #178
    Senior Member Kopachris's Avatar
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    Quote Originally Posted by Meaghan View Post
    Hi folks!
    I'm looking at this thread because I just started taking an astronomy course at my college. We have a little planetarium and we all sat in it this evening while the professor showed us some constellations and how the sun's arc across the sky changes with the seasons and why Mercury appears to move back and forth and some coordinate systems for locating things in the sky and some other stuff. I definitely didn't take it all in, but it's exciting! So I'm just poking my head in to say that I'm looking forward to gaining some knowledge of astronomy and getting involved with this thread!
    You sound like you're in for a fun class! If you have any questions, I'd love to show off a little.
    Nothing happens to me. -- Famous last words of Dr. John H. Watson

  14. #179
    Senior Member kv466's Avatar
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    Meaghan, you just started getting into all my favorite things, didn't you?! Steel-string acoustic,...astronomy... If you listen to my show, which is a few posts back, it'll tell you exactly how the sky is positioned just after sunset. Happy to see you enjoying the cosmos!

  15. #180
    Senior Member Lunasong's Avatar
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    Last night I was out around ten PM to pick out the Summer Triangle (Deneb, Vega, Altair) and also spotted the "teapot" of Sagittarius, an unusual sight for me because of its low position in the south, where I experience the most light pollution.

    I was up at six this AM to look at the sky before it started getting light. I was rewarded by the sight of bright Jupiter high and almost overhead in the constellation Taurus. Aldebaran and the Pleiades were clearly visible.

    The easily recognizable Orion was down below and a bit to the south (right).
    The Moon is in Gemini. I could easily pick out Castor and Pollux as in the chart above.
    Venus is still shining brightly closer to the horizon.

    It was extremely clear last night as we have unusually low humidity, which makes for more visible stars!

    ps: happy birthday, kv466!
    "To be a musician is a curse. To NOT be one is even worse." Jack Daney

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