Color Indicies

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

DannyBoy

"DannyBoy" wrote in message
...
Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute
color
magnitude measured at UV minus absolute color magnitude measured at blue)
if
there is no absorption?
Or is this always the case?

Getting the colour in the observed system is only the first step. Your
system of filters and detectors has to be calibrated against a standard set
of filters by observing standard stars (lists are available in the
astronomical literature). You also have to correct for colour effects and
absorption in Earth's atmosphere. Finally, you have to make absolutely sure
there is no infrared light leaking into your U-band filter observation--this
can be a common fault, especially when using CCDs.

Typically, the observed colour is about a magnitude more positive than the
calibrated standard colour U-B.

The atmospheric extinction depends on the zenith distance of the star at the
time of observation.

There is also a complicating effect because the coefficients vary depending
on the intrinsic colour of the star, because the average wavelength of a
U-band is different for a hot star and a cool star, and atmpospheric
extinction varies rapidly with wavelength.


Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?


Assuming these things, yes, colour wold be independent of distance, but once
you look at stars more than about 50pc away, there will always be some
absorption and reddening by interstellar dust.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

"DannyBoy" wrote in message
...
Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute
color
magnitude measured at UV minus absolute color magnitude measured at blue)
if
there is no absorption?
Or is this always the case?

Getting the colour in the observed system is only the first step. Your
system of filters and detectors has to be calibrated against a standard set
of filters by observing standard stars (lists are available in the
astronomical literature). You also have to correct for colour effects and
absorption in Earth's atmosphere. Finally, you have to make absolutely sure
there is no infrared light leaking into your U-band filter observation--this
can be a common fault, especially when using CCDs.

Typically, the observed colour is about a magnitude more positive than the
calibrated standard colour U-B.

The atmospheric extinction depends on the zenith distance of the star at the
time of observation.

There is also a complicating effect because the coefficients vary depending
on the intrinsic colour of the star, because the average wavelength of a
U-band is different for a hot star and a cool star, and atmpospheric
extinction varies rapidly with wavelength.


Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?


Assuming these things, yes, colour wold be independent of distance, but once
you look at stars more than about 50pc away, there will always be some
absorption and reddening by interstellar dust.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

wrote in message . ..
DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

Excuse me, could one of you learned gentlemen point me to a site ref
spectrum and factors affecting it? Intuitively I suspect that red
shift has a cause other that velocity away from us, and some of the
clues might be found there.
thank you

Jim G

wrote in message . ..
DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

Excuse me, could one of you learned gentlemen point me to a site ref
spectrum and factors affecting it? Intuitively I suspect that red
shift has a cause other that velocity away from us, and some of the
clues might be found there.
thank you

Jim G

Jim Greenfield writes:

DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

Excuse me, could one of you learned gentlemen point me to a site ref
spectrum and factors affecting it?

I need to know what a "site ref spectrum" is before I can point you to
it.

Intuitively I suspect that red
shift has a cause other that velocity away from us, and some of the
clues might be found there.

There are gravitational redshifts, but that doesn't mean that that
velocity-induced redshifts don't occur.

Jim Greenfield writes:

DannyBoy writes:

Can I assume that U-B (apparent color magnitude measured at UV minus
apparent color magnitude measured at blue is equal to Mu-Mb (absolute color
magnitude measured at UV minus absolute color magnitude measured at blue) if
there is no absorption?
Or is this always the case?

Mu-Mb is of course independent of distance but is U-B independent of
distance assuming perfect vaccum and no absorption?

The distinction between apparent and absolute magnitude is based on
distance. The inverse square law is wavelength independent, thus the
distance modulus is the same for all wavelengths. If the vacuum is
perfect, then their is nothing else to affect the color.

Excuse me, could one of you learned gentlemen point me to a site ref
spectrum and factors affecting it?

I need to know what a "site ref spectrum" is before I can point you to
it.

Intuitively I suspect that red
shift has a cause other that velocity away from us, and some of the
clues might be found there.

There are gravitational redshifts, but that doesn't mean that that
velocity-induced redshifts don't occur.

wrote in message . ..
Jim Greenfield writes:
Excuse me, could one of you learned gentlemen point me to a site ref
spectrum and factors affecting it?

I need to know what a "site ref spectrum" is before I can point you to
it.

I have in mind experiments measuring white-light velocity, and then
the velocity of the different component colours.
Experiments of light velocity in a vacuum, with the source separated
by different mediums (glass, water) (Say light passes through 100
meters of water, then 5mm glass, then into vacuum, is velocity same as
from that source generating in the vacuum)

Intuitively I suspect that red
shift has a cause other that velocity away from us, and some of the
clues might be found there.

There are gravitational redshifts, but that doesn't mean that that
velocity-induced redshifts don't occur.

Agreed/ thanks

Jim G