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Story and Images By Dr. Gregory D. Wirth
February 5, 2007 presentations to 8th & 10th grade students
at Parker School
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| Photo: Dr. Greg Wirth, astronomer at Keck
Observatory, speaking to students at Parker School. |
Aloha! Thanks for welcoming me to Parker School today. My name
is Dr. Greg Wirth and I‘ve been a “support astronomer” at Keck
Observatory for the past nine years. If you think of Keck as
a full-service observatory, you could say I’m a glorified customer-service
representative. I'm responsible for helping the visiting astronomers
--- who fly out here from all over the world just to observe
through our telescopes one or two nights a year --- to gather
the data they need to answer whatever scientific questions
they're pursuing.
I grew up in a place far away from here: Midland, Michigan.
Midland is located in the middle of the lower peninsula of
Michigan, and Midland is a lot cloudier and colder than Hawai‘i.
In spite of the clouds and the freezing winter temperatures,
I got interested in astronomy thanks to people who shared their
enthusiasm for astronomy with me, and I’m happy to repay them
by sharing my excitement about science with you.
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| Photo: Greg Wirth as an 11th grader at
Dow High School in Midland, Michigan. |
When I left Michigan, I started heading west, first to the
Chicago area to attend college and then to Santa Cruz, California,
to earn a doctorate. It takes a while, sometimes many years,
to become a scientist. But I’ve found it’s worth the time,
because along the way you get to go wonderful places, do fun
things, and work with incredible people.
I study galaxy clusters, which are the largest groups of matter
out there. Galaxy clusters are made up of hundreds or thousands
of galaxies, and they are the largest organized structures
in the universe.
Einstein taught us that matter warps space. Warped space acts
like a lens, so that distant galaxies in the background get
distorted and magnified when viewed through an intervening
cluster of galaxies. These “gravitational lenses” help astronomers
to study the most distant galaxies in the universe.
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| Photo: Gravitational lens in galaxy cluster
Abell 1689 from Hubblesite.org. |
The important thing is that you don’t need to be
Einstein to be a scientist. There are many different
fields open to you and many different ways to get there. Allan
Honey, who volunteered to act as my “wireless mouse” for this
presentation today, is a computer scientist who works at Keck
Observatory. We have engineers, computer scientists, astronomers,
and people from a whole gamut of different fields working at
Keck.
So what is astronomy? Basically, it’s the study of universe.
There are three big questions that astronomers study: what’s
out there, what is the origin of the universe, and how does
the universe function? We also try to discover if we are alone — or
if there is intelligent life on other worlds.
How do we do this? Biologists and chemists work in laboratories,
while astronomers use telescopes. There is a fundamental difference
between astronomy and these other fields. We cannot take a
star or galaxy into the lab to analyze. Instead, we must be
patient and wait for phenomena to occur naturally. If we want
to see how a supernova explodes, we must wait — sometimes for
centuries -- for one to occur nearby so that we can look at
it using our telescopes and try to understand what is happening.
Light is the astronomer’s pathway to knowledge — astronomers
gather and analyze it to learn about the universe.
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| Image: Earth's atmosphere is approximately
78% nitrogen, 21% oxygen. |
When we think of light, we usually think only of the visible
light that we see with our eyes; however, the electromagnetic
spectrum consists of different types of light, ranging from
microwaves (lowest energy) to gamma rays (highest energy).
Radio waves are at the low end of the spectrum. This means
that when we listen to the radio, we’re actually listening
to light waves. Our eyes cannot actually see most of the spectrum.
Only small bands of light make it through the Earth’s atmosphere,
which is unfortunate for astronomy but very fortunate for life
on earth: we wouldn’t last long if Earth’s atmosphere didn’t
block all the harmful X-rays and gamma rays that bombard the
planet daily. Visible light is one of the kinds of light that
can pass through the Earth’s atmosphere. Human eyes have evolved
to see light where the atmosphere is transparent, as it is
in the visible part of the spectrum. (slide)
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| Photo: Infrared view of a man’s head. Everyday
objects emit thermal radiation, and we equate infrared
light with heat. |
Other creatures see the Universe differently. For example,
rattlesnakes see infrared light — or the generated heat which
is radiated off of objects, including other life forms. Using
an infrared camera, or “snake vision,” as I call it, astronomers
can see the glowing gases in space which form new stars or
get ejected from exploding stars. Using an infrared filter
to view the Moon makes it appear reddish.
Depending on how you look at it, or which part of the electromagnetic
spectrum you focus on, the universe looks very different. We
typically see the visible light of the sun. But if we view
the sun in the infrared part of the spectrum, we can see sunspots;
in the ultraviolet, the sun looks very uniform; and in the
far-ultraviolet (x-rays), the Sun’s corona — a high-temperature “halo” around
the sun — becomes visible.
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| Photo: Each Keck telescope has a mirror
which consists of 36 segments, each measuring 1.8 meters,
which are shaped like hexagons and assembled in the shape
of a honeycomb to act as a single large mirror. |
Why do we use segmented mirrors? One main reason is that it
is too difficult to manufacture and transport one large mirror.
Also, it is easier to clean and recoat individual segments
than it is to maintain one single large mirror. Because we
have replacement segments, there is little “down time” at Keck
while segments are being maintained. We just install a spare
while the segments are cleaned and recoated. The joints between
the segments are barely perceptible. We use a sophisticated
system to keep the individual mirror segments aligned with
one another — this alignment technology is one of the operating
systems that we have perfected at Keck Observatory.
So, how big would the moon look if you saw it at the focus
of a Keck telescope? It would be “gi-normous,” or several feet
in diameter.
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| Photo: from National Geographic magazine
showing one of my colleagues standing at the side of
the telescope with the full moon in view. |
Astronomers are able to analyze the light from objects in two
ways. First, we can collect images. Images help astronomers
to learn more about the structure and brightness of celestial
objects (images reveal their color, size, brightness, and shape).
But a simple picture doesn’t tell us the whole story.
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| Photo: Left, nearby comet; right, galaxy
60 million light years away. |
From looking at an image, we can’t measure the distance of
the light source. Two images can look similar even though their
sources are very different. For example, a galaxy (right side
of photo to the left) that is 60 million light years away may
look similar to a comet (left side of the photo) that is only
one-millionth of a light year away. (A light-year is the distance
that light, moving at 299,793 kilometers per second, travels in
one year; it equals 5.88 trillion miles. Light years are commonly
used in astronomy to describe large distances.) Although the
images of two objects may look similar through the telescope,
they may be vastly different. Astronomers have an amazing tool
that allows them to determine the temperature, composition,
and velocity of objects in space: and that is the spectrum of
light.
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| Photo: A moonbow in the foreground and
a rainbow in the background. |
Isaac Newton was the first to determine that light consists
of different colors, or a spectrum. We see a spectrum every
time we look at a rainbow. The raindrops act like a prism to
break up light into its component colors. In Hawai‘i we sometimes
have the privilege of seeing not only rainbows, but also moonbows.
Moonbows, like rainbows, happen when there are both light (in
this case, moonlight) and rain. According to my Keck colleague
Allan Honey, a great place to see a moonbow is the neighborhood
called Kamuela View Estates on the west side of Waimea, when
the clouds are below you and the moon is rising.
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| Photo: Vislble star color denotes temperature. |
When we look at stars, we can all see that they are different
colors. This is because they have different temperatures. An
object that is cooler produces more red light, so a cool star
looks very red. A hot star looks very blue. Bigger stars are
more likely to be hotter, and therefore bluer. By analyzing
the spectrum of a star, we can directly measure its temperature
and make conclusions about its probable mass.
Spectra also reveal the composition of a star. Each element
has a distinct spectral fingerprint related to the number of
electrons in that atom. Hydrogen, for example, is a simple
element — one electron and one proton -- with a simple spectrum.
The fact is that every hydrogen atom in the universe glows
the same way, or emits the same spectrum. Nitrogen has a more
complex spectrum because it has more electrons and protons.
And every nitrogen atom in the universe exhibits this same
spectrum. This is a really powerful concept which makes it
possible for astronomers to identify which elements comprise
a star, a gas cloud, and other celestial objects. We can even
study gas clouds between us and the object we are trying to
observe. These intervening gas clouds absorb certain wavelengths
of light, and so we can analyze their spectra to see what they
are made of.
View a video clip of
Dr. Greg Wirth demonstrating the spectral fingerprints of hydrogen
and nitrogen to Parker School 10th graders (requires
Quicktime player).
Here’s a concrete example from within our own Solar System:
by analyzing the spectrum of light from Mars, we know that
about 95% of the Martian atmosphere consists of carbon dioxide
gas. Like atoms, each kind of molecule also has its own “fingerprint”:
carbon dioxide has a unique signature and that’s what we see
in spectra of Mars.
Finally, spectra are used to determine the velocity of objects
in space. You‘re probably familiar with the Doppler Effect,
even if you don’t know it by name. When an airplane is coming
toward you, the pitch of its engines is higher, and when it
moves away from you, the pitch is lower. The Doppler Effect
also works with light. Higher frequency light is bluer, so
that a star moving toward the observer looks bluer. A star
moving away from the observer has a lower frequency, and the
light it emits is redder.
Astronomers are using the Doppler Effect to detect otherwise “invisible” planets
orbiting around nearby stars. The light from a star changes
due to the gravitational pull exerted on the star by an unseen
planet. The “wobble” or color shift of a star with a planet
orbiting it happens very regularly, allowing astronomers to
detect planets around a star. When the planet is moving towards
us, the star moves away from us and thus its light becomes
very slightly shifted towards the red.
Conversely, when the planet moves away from us, the star moves
towards us and the light is shifted slightly toward the blue.
Using this technique, astronomers worldwide have discovered
209 planets since 1995 — most of which have been discovered
using data from the HIRES spectrograph on the Keck I telescope.
We’re very proud that our observatory has made this contribution
to human knowledge.
As I mentioned at the outset, there are three big questions
that astronomers are trying to answer about the universe: How
big is it? How did it begin? And what is its fate?
The modern answers to all of these questions begin with observations
made by the American astronomer Edwin Hubble, for whom the
Hubble Space Telescope is named, Hubble observed the spectra
of many nearby galaxies, and then he used these spectra to
measure the velocities of these galaxies. Hubble found a curious
relationship: almost all of the galaxies were receding away
from us, and the galaxies that were further away were moving
away from us more quickly. Hubble was the first astronomer
to discover that the universe is expanding!
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| Image: Raisin cake model of an inflating
universe. |
At this point, you may be thinking that if all of the galaxies
in the universe are moving away from us, that means we must
be at the center of the expansion. Actually, that’s not the
case. Imagine that the universe is a cake and the
galaxies are raisins within the cake. As the cake bakes it
expands in all directions and every raisin in the cake gets
further apart from the other raisins. Similarly, every galaxy
in our universe is getting further away from every other galaxy.
Here’s one final point to consider: how can we study stars
and galaxies which are billions of years old when humans are
only alive for, at most, a hundred years? The answer is that
telescopes serve as time machines. Since light travels at a
finite speed, we see things not as they are now, but as they
were when the light left them some time ago. The further away
in space something is from us, the further back in time we
must look to see it.
Are you seeing me as I am now? No -- you see the light that
was emitted from me some fraction of a nano-second ago. Similarly,
we see the sun as it was eight minutes ago, Pluto as it was
4 hours ago, the nearest star as it was 4.3 years ago, and
the nearest large galaxy, Andromeda, as it was 2.9 million
years ago. The light gathered by our telescopes allows us to
determine how things looked billions of years ago -- more than
4.5 billion years ago, before the origin of Earth — and even
further back to the origins of the universe itself.
Something like 99% of astronomers agree that the Big Bang is
a good theory which accounts for the origin of our universe.
The Keck telescopes can see almost all the way back to the
Big Bang, roughly 14 billions year ago. In fact, the “snow” that
you see on your TV set when it’s not receiving a signal from
a station is, in fact, partially made up of light which was
emitted in the Big Bang.
Pretty cool, eh?
View a brief Journey
through the Universe video clip, narrated by Dr.
Greg Wirth (4.6 Mb, requires Quicktime player).
View Light, Optics, and Spectra in
Astronomy, courtesy of Dr. Shui Hung Kwok of Keck Observatory,
which demonstrates some of the concepts discussed above. Refer
to the following captions as you scroll through the animation:
- Light is emitted by a light source, such as a light bulb,
in circular waves.
- A lens gathers light.
- A lens focuses light to produce an image on the opposite
side from the source. Note that you can use the mouse to
move the source, and the image position will change.
- A star emits light in waves also. Although they are always
circular, at large distances they can be considered flat
or "plane" waves.
- A concave mirror focuses light from a star to a point.
- Telescopes are generally designed with a "secondary" mirror
to refocus the image of a star to a place where we can
mount an instrument to analyze the light. In this case,
light reflected off the secondary mirror passes through
a hole in the primary mirror. We call this a Cassegrain
telescope design.
- We can also use a third or "tertiary" mirror to reflect
the light to the side of the telescope.
- The tertiary mirror can redirect the light to either
side of the telescope.
- As light passes through empty space it is virtually undisturbed,
but the turbulence in Earth's atmosphere distorts the incoming
light. Instead of a sharp image of the star, we see a blurry
one.
- This shows the view from Earth of a distant star with
a planet going around it. As the planet orbits the star,
it causes the star to wobble slightly in a very regular
pattern every time the planet completes one orbit. This
wobble causes the light from the star to be shifted (Doppler
shift). By taking a spectrum of the star, we can measure
this wobble and thus deduce the presence of the planet
even if we can't see it.
- A heavier planet produces a greater shift in the star's
light. By measuring the size of the shift, we can determine
the mass of the planet.
- Top-down view of the planetary system to show that the
orbit is actually circular.
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