Occultation Question

[The Code]

Hi

http://en.wikipedia.org/wiki/Occultation

Does this ever happen and nothing in the solar system was the cause? Including the Oort cloud.

http://images.google.co.uk/imgres?imgur ... X%26um%3D1
Thanks

[bystander]

Not really sure what you are asking, but occultation is one of the methods used to detect extra-solar planets.

[The Code]

Do distant stars dim, and go out for no apparent reason? Then shine again minutes later? If so, how regular is it?

[Chris Peterson]

Do distant stars dim, and go out for no apparent reason? Then shine again minutes later? If so, how regular is it?

Never. Stars can be occulted by asteroids, but this is predictable. There are also observing programs looking for small intensity fluctuations in stars caused by MACHOs (typically brightening, not dimming), but fluctuations are rare, unpredictable, and unrepeatable, making analysis very difficult and interpretation variable.

[The Code]

Hi Chris



Have stars gone out for no reason yes or no? Vary rare means they have.

[Chris Peterson]

Have stars gone out for no reason yes or no? Vary rare means they have.

Not that I'm aware of. Stars have shown small unexpected increases in brightness, which are explainable in several ways, but for which there is no consensus explanation. As noted, stars can and occasionally are occulted by objects in the Solar System.

[The Code]

What colour comes after blue?

[Chris Peterson]

What colour comes after blue?

What do you mean by "after"? Towards the longer wavelengths is green, towards the shorter is purple. Go too far either way and you get out of the visible range, at which point "colour" doesn't mean much.

[The Code]

I have never seen a green star, or a purple star. But I have seen a red star, and a blue star.

I,ve heard of stars that maybe black. But people call them other things.

[neufer]

What colour comes after blue?

What do you mean by "after"? Towards the longer wavelengths is green, towards the shorter is purple.

Has INDIGO gone the way of PLUTO
There should be more VIOLETS in the spectrum
Sincerely, ROY G. BIV

Indigo was defined as a spectral color by Sir Isaac Newton when he divided up the optical spectrum, which has a continuum of wavelengths. He specifically named seven colors primarily to match the seven notes of a western major scale, because he believed sound and light were physically similar, but also to link colors with the (known) planets, days of the week, and other lists that had seven items.

The human eye is relatively insensitive to hue changes in the wavelengths between blue and violet, where Newton defined indigo to be; most individuals do not distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, hold that indigo should not be regarded as a color in its own right, but merely as a hue of blue or violet.

Indigo (spectral approximation) (#6600FF)
Indigo (web color) (#4B0082)

Wavelength 420–450 nm

sRGBB (r, g, b) (75, 0, 130)

HSV (h, s, v) (275°, 100%, 51%)

<<Indigo is the color on the electromagnetic spectrum between about 420 and 450 nm in wavelength, placing it between blue and violet. Although traditionally considered one of seven divisions of the optical spectrum, modern color scientists do not usually recognize indigo as a separate division and generally classify wavelengths shorter than about 450 nm as violet.

Like violet, whether indigo is considered a shade of purple depends on context. Common English usage defines purple as any color between red and blue whereas in color theory, purple is defined as any non-spectral color between violet and red. Thus indigo and violet fit the common but not the color theory definition of purple.

One can see spectral indigo by looking at the reflection of a fluorescent tube on the underside of a non-recorded compact disc. This occurs because the CD functions as a diffraction grating, and a fluorescent lamp generally has a peak at 435.833 nm (from mercury), as is visible on the fluorescent lamp spectrum.

The first recorded use of indigo as a color name in English was in 1289.

India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the "dye", which was indikon. The Romans used the term indicum, which passed into Italian dialect and eventually into English as the word indigo.

Color scientists typically divide the spectrum at about 450 nm between violet and blue, with no indigo. Others continue to accept it, as it has been accepted traditionally as one of Newton's named colors of the spectrum along with red, orange, yellow, green, blue, and violet.>>

[Chris Peterson]

I have never seen a green star, or a purple star. But I have seen a red star, and a blue star.

Indeed, there are no green stars. That's because stars are substantially continuum sources following the rules of blackbody radiators. The shape of such a spectral curve doesn't allow for any temperature that produces light we see as green, even though the peak wavelength can be in the green part of the spectrum. Our own sun has its peak output in the green-yellow area, although it appears white to our eyes.

Just consider what a body does as you heat it: it goes from black (no visible radiation), to red, yellow, white. Very hot stars can shift the peak far enough that we see them as blue or purple (although their peak is probably in the UV). There are a few special cases where the color of a star can be modified by absorption in their envelopes (carbon stars) or by absorption or scattering in intermediate dust or molecular clouds, but those are pretty uncommon.

[Chris Peterson]

Has INDIGO gone the way of PLUTO?
There should be more VIOLETS in the spectrum!
Sincerely, ROY G. BIV :(

I still teach students that Pluto is a planet, but I generally leave out indigo and describe the visible spectrum going from blue to violet. Visual color sensitivity isn't very good at short wavelengths, and I've always considered indigo to be a pretty artificial addition- not to mention that as a color it isn't very well understood by most people. I only include it when using the above mnemonic.

[The Code]

Is that as far as you know Chris? or is there more?

[neufer]

Just consider what a body does as you heat it: it goes from black (no visible radiation), to red, yellow, white.

• BREDORANGEYELLOWHI
  
  • Forbidden Planet (1956)
  Commander John J. Adams: And if you don't do something
  about it soon...it's going to be coming right through that door.
  
  Dr. Edward Morbius: Solid Krell metal, twenty six inches thick.
  
  Commander John J. Adams: Look at your gauges. Look!
  That machine is going to supply your monster...
  with whatever amount of power it requires to reach us.
  
  Look now! Red-hot.
  Soon it'll be white-hot.
  Then it'll soften and melt.
  
  http://www.youtube.com/watch?v=GQwmKClCAbQ
  
  Dr. Edward Morbius: My evil self is at that door, and I have no power to stop it!

[neufer]

Has INDIGO gone the way of PLUTO?
There should be more VIOLETS in the spectrum!
Sincerely, ROY G. BIV

I still teach students that Pluto is a planet,

Why?

I generally leave out indigo and describe the visible spectrum going from blue to violet. Visual color sensitivity isn't very good at short wavelengths, and I've always considered indigo to be a pretty artificial addition- not to mention that as a color it isn't very well understood by most people. I only include it when using the above mnemonic.

<<Violet is a spectral color (approximately 380-420 nm), of a shorter wavelength than blue,
while purple is a combination of red and blue or violet light.
The purples are colors that are not spectral colors – purples are extra-spectral colors.

In fact, purple was not present on Newton's color wheel (which went directly from violet to red), though it is on modern ones, between red and violet. There is no such thing as the "wavelength of purple light"; it only exists as a combination.

On the CIE xy chromaticity diagram, violet is on the curved edge in the lower left, while purples are the straight line connecting the extreme colors red and violet; this line is known as the line of purples, or the purple line.Pure violet cannot be reproduced by a Red-Green-Blue (RGB) color system, but it can be approximated by mixing blue and red. The resulting color has the same hue but a lower saturation than pure violet.>>

[Chris Peterson]

I still teach students that Pluto is a planet,

Why?

Well, because it is.

The purples are colors that are not spectral colors – purples are extra-spectral colors.

I normally treat "purple" and "violet" as synonymous, which is true in most, although not all contexts.

Spectral colors only represent a small part of the human gamut.

[Chris Peterson]

Is that as far as you know Chris? or is there more?

I said "Not that I'm aware of", which pretty much means "as far as I know". Feel free to review the literature in search of an anomalous dimming; maybe you can find something. The problem is that anything witnessed visually is going to be an anecdotal report. You'd want something recorded instrumentally, and most stars simply aren't under long term observation.

[neufer]

The purples are colors that are not spectral colors – purples are extra-spectral colors.

I normally treat "purple" and "violet" as synonymous, which is true in most, although not all contexts.

Spectral colors only represent a small part of the human gamut.

I was assuming that the question: "What colour comes after blue?" and answer:
"Towards the longer wavelengths is green, towards the shorter is purple."
was referring specifically to spectral/rainbow colors.

Otherwise, perhaps, you should have responded: "Towards redder colors is purple."

[Are Infrascarlet & Ultrapurple going to be O.K. spectral substitutes?]

• • The Color Purple
  Shug: I think it pisses God off when you walk by the color purple in a field and don't notice it.
  
  Celie: You saying it just wanna be loved like it say in the bible?
  
  Shug: Yeah, Celie. Everything wanna be loved. Us sing and dance, and holla just wanting to be loved.
  Look at them [olive] trees. Notice how the trees do everything people do to get attention... except walk?

[THX1138]

Hi Chris
I was going over / looking at the / your cloudbait link and going over some of the past occulation graphs for Pluto shown there and i was wondering why the graphs seem to jump around, that is rather than flowing evenly. Is this due to star / sun flares that would do that, mountains on Pluto or just our earth atmosphere that causes it?
Thank you

[Chris Peterson]

Hi Chris
I was going over / looking at the / your cloudbait link and going over some of the past occulation graphs for Pluto shown there and i was wondering why the graphs seem to jump around, that is rather than flowing evenly. Is this due to star / sun flares that would do that, mountains on Pluto or just our earth atmosphere that causes it?

Mainly, it is just statistical noise. Except for the dip in brightness during the occultation, the source intensity is very uniform. But a perfectly uniform light source doesn't spit out photons like clockwork, but in a temporal pattern described by a Poisson distribution. This means you get little bunches of photons, and then little pauses. The more photons you get, the closer you get to knowing the true intensity, and the less variation seen (the error is equal to the square root of the signal; if the source intensity is 100 photons per second, there is a 95% chance it will actually produce between 90 and 110 photons during any given second). This means that you need a lot of photons to get a very uniform signal. That's why astroimagers like bright targets, big telescopes, and long exposures- all of which boost the photon count. But we photometricists don't have it so easy. Targets are often dim (as here, with Pluto and a chance background star), and exposure times often small, since we need to preserve fine time resolution to record the event. So in the case of the data you were looking at, most of the variation is simply caused by the sqrt(N) noise on a weak signal. There may also be a small amount of variation from atmospheric effects (the Earth's atmosphere), but the photometric measurement technique is designed to minimize that.