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Astronomical
images
How they are made,
what we can learn from
them?
Modern telescopes
all have instruments
attached, starting
with cameras.
Let’s look at some
astronomical camera
images:
How are these images made?
Pixels and your camera
Pixels: “picture elements”, cen
be seen when we zoom in on
a digital image.
Each pixel represents the light
that fell on it. Computers
handle this as a number.
(we will get to color later…)
A digital, or CCD, camera
can be used to measure the
light in each pixel
If you can’t measure it,
it’s not science!
The M&M model CCD:
Count the M&Ms to measure
how much light struck the pixel
What can we learn from measuring
the light in each pixel?
Suppose we
want to learn
about the stars
in this “nest”
Let’s see how
some physics
can help
understand
them
Two very hot objects,
one at 6000 degrees,
one at 4000 degrees
The sun’s energy, plotted
the same way: what’s its
temperature?
(Can we do this with just
a few points?)
Conclusion: If we can measure the brightness
of a star in just 2 different colors, we can
measure its temperature
Filters: only transmit a narrow range of color
So we image stars through different filters to measure
their temperature.
And we also combine images taken with different filters,
and assign colors to each filter to get color pictures:
Results for Stars within this cluster:
We plot temperature (from different color filters) against
brightness *, ( luminosity) for stars within a cluster
* Count those M&Ms!
From plots like this we can deduce
a great deal about these stars.
Through this plot of stellar brightness vrs
temperature, we have a snapshot of a star’s life
cycle
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Temperature (from color)
A Star’s Life Cycle
Our plot of temperature against
brightness is called an HR
diagram.
If we could watch over millions
of years, we would see their life
cycles as stars change:
•Stars form in clouds of gas, settle onto the Main Sequence
•Massive, hottest stars live short lives, explode as supernova
•Intermediate mass stars like our sun live for billions of years
•Low mass, coolest stars last even longer than our sun
But imaging only tells us part of the story:
What’s the wispy material ? What are stars
composed of? How fast are bodies moving? How big
are they?…
There is only so much physics that
images taken with filters can tell you
about your objects,
Which brings us to “spectroscopy”
We could say that imaging with filters is a
version of spectroscopy, which is a
means of studying objects as a function
of the details of way the light from it
varies with color.
Spectroscopy
Have you seen the same thing from light off of a music CD
or out of a prism
But when we look at a star we see
Alpha Bootes
Looking at something familiar
Lights in a parking lot in Tucson:
“Neon”, car lights, HP sodium, metal halide
Spectral Analysis
So the spectra reveal details about the light source.
We can use that same approach to study planets, stars,
and galaxies.
We can deduce the temperature and abundances of
various chemical elements in the outer atmosphere of
a star.
Theorists predicted that when the universe was forming
there was little more than H, He, Li, Be, … - the
smallest, lightest elements.
Observers are confirming that the oldest stars were
much like that - more recently formed ones have
more “heavy” elements.
We can also measure how fast objects are moving towards or away from
us, like the VERY distant Quasars shown above;
for some stars, we can use the same technique to estimate how fast they
are rotating
Conclusion: different instruments on telescopes
reveal different pieces of the puzzle!
(And we haven’t even mentioned how
astronomer often use observations taken by
satellites in orbit that cover wavelengths of
light that cannot be observed from the
ground!)