Download 2.1 Hubble Space Telescope

2.1 Hubble Space Telescope
More than a decade after launch, The Hubble Space Telescope (HST) continues to
produce excellent scientific results and stunning imagery.
In the past year, the active instruments were the infrared camera NICMOS, the
Wide Field Planetary Camera 2 (WFPC2) and the Fine Guidance Sensors (FGS).
They are all performing nominally. The Space Telescope Imaging Spectrograph
(STIS), the only spectrograph on board the HST, unfortunately ceased operating in
August 2004, and the Advanced Camera for Survey ceased operations in January
2007. We are now working towards Servicing Mission 4, currently planned for
Summer 2008. During this mission, two new instruments will be installed: the Wide
Field Camera 3 (WFC3) and the Cosmic Origin Spectrograph (COS). Repairs will
be attempted to restore both STIS and ACS.
Observations with HST have impacted every area of current astronomical research.
A few highlights of the results obtained during the year 2007 are presented here.
Astronomers have long thought that globular star clusters had a single star-formation
episode early in their lives and then settled into a quiet existence. Observations by
the Hubble Space Telescope, however, are showing that this idea may be too simple.
The Hubble analysis of the massive globular cluster NGC 2808 provides evidence
that star birth occurred again and again, with three generations of stars forming very
early in the cluster’s life.
Globular clusters are the homesteaders of our Milky Way Galaxy, born during our
galaxy’s formation. They are compact swarms of typically hundreds of thousands of
stars held together by gravity. The standard picture of a globular cluster is that all of
its stars formed at the same time, in the same place, and from the same material, and
they have co-evolved for billions of years. This is the cornerstone on which much of
the study of stellar populations has been built. So astronomers were very surprised
to find several distinct populations of stars in NGC 2808. All these stars were born
within 200 million years, very early in the life of the 12.5-billion-year-old massive
cluster.
Finding multiple stellar populations in a globular cluster so close to home has
deep cosmological implications, say the researchers. The astronomers used Hubble’s
Advanced Camera for Surveys to measure the brightness and colour of the cluster
stars. Hubble’s exquisite resolution allowed the astronomers to sort out the different
stellar populations. The Hubble measurements showed three distinct populations,
with each successive generation appearing slightly bluer. This colour difference
suggests that successive generations contain a slightly different mix of some
chemical elements.
One possibility is that the successively bluer colour of the stellar populations
indicates that the amount of helium increases with each generation of stars. Perhaps
massive star clusters like NGC 2808 hold onto enough gas to ignite a rapid succession
of stars. The star birth would be driven by shock waves from supernovae and stellar
winds from giant stars, which compress the gas and make new stars. The gas would
be increasingly enriched in helium from previous generations of stars more massive
than the Sun.
Astronomers commonly assume that globular clusters produce only one stellar
generation because the energy radiating from the first batch of stars would clear out
most of the residual gas needed to make more stars. But a hefty cluster like NGC
2808, which is two to three times more massive than a typical globular cluster, may
have enough gravity to hang onto that gas, which has been enriched by helium from
the first stars. Of the approximately 150 known globular clusters in our Milky Way
Introduction
Multiple stellar generations
in a globular cluster
For further information, see http://sci.esa.int/hubble
Forming stars in the Small
Magellanic Cloud
Figure 2.1.1. In a simplified version of the Hertzsprung-Russell
diagram, this graph schematically plots the brightness of the
stars in globular cluster NGC 2808 (along the vertical axis)
against stellar colour and temperature (along the horizontal
axis). The cooler a star is, the redder it appears, and it
diminishes in brightness. The bluer stars are to the left; redder
stars are to the right; the brightest stars are near the top. The
three curves represent the three different populations of stars
that are present in NGC 2808.
Figure 2.1.2. Image of the Small Magellanic Cloud starforming region NGC 602, taken with the Advanced Camera
for Surveys. Filters that isolate visible and infrared light were
combined with a filter that samples the hydrogen and nitrogen
emission from the glowing clouds. Star formation started at
the centre of the region, and propagated outwards. Newborn
stars have been found along the ‘dusty’ ridges.
Galaxy, NGC 2808 is one of the most massive, containing more than 1 million
stars.
Another possible explanation for the multiple stellar populations is that NGC 2808
may only be masquerading as a globular cluster. The stellar grouping may have been
a dwarf galaxy that was stripped of most of its material due to gravitational capture
by our galaxy.
Although the astronomers’ search is only in its infancy, multiple stellar populations
may be a typical occurrence in other massive clusters.
Forming stars in the Small
Magellanic Cloud
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A new generation of bright, blue, newly-formed stars have been found blowing a
cavity in the centre of a star-forming region in the Small Magellanic Cloud.
At the heart of the star-forming region lies star cluster NGC 602. The high-energy
radiation blazing out from the hot young stars is sculpting the inner edge of the outer
portions of the nebula, slowly eroding it and eating into the material beyond. The
diffuse outer reaches of the nebula prevent the energetic outflows from streaming
away from the cluster.
Ridges of dust and gaseous filaments are seen towards the northwest (in the
upper-left part of Fig. 2.1.2) and towards the southeast (the lower right-hand corner).
Observations taken with the Spitzer Space Telescope in infrared have shown that
star formation is presently ongoing along the ridges of the star-forming region,
and several ‘just born’ stars are found there. Elephant trunk-like dust pillars point
towards the hot blue stars and are tell-tale signs of their eroding effect. In this region
it is possible with Hubble to trace how the star formation started at the centre of the
cluster and propagated outward, with the youngest stars still forming today along the
dust ridges.
The Small Magellanic Cloud is roughly 200 000 light-years from Earth. Its
proximity to us makes it an exceptional laboratory for performing in-depth studies
of star formation processes and their evolution in an environment slightly different
from our own Milky Way.
Dwarf galaxies such as the Small Magellanic Cloud, with significantly fewer stars
compared to our own galaxy, are considered to be the primitive building blocks of
larger galaxies. The study of star formation within this dwarf galaxy is particularly
interesting to astronomers because its primitive nature means that it lacks a large
percentage of the heavier elements that are forged in successive generations of stars
through nuclear fusion.
The galaxy I Zwicky 18 appears to look older the more astronomers study it. The
galaxy’s youthful appearance was identified some 40 years ago through observations
at the Palomar Observatory. Those studies showed that the galaxy erupted with
star formation billions of years after its galactic neighbours. Galaxies resembling
I Zwicky 18’s youthful appearance are typically found only in the early Universe.
Astronomers were thrilled that a newly-forming galaxy like I Zwicky 18 could be
studied nearby to learn about galactic evolution, which is normally only observable
at great distances.
New Hubble data have quashed that possibility. The telescope found faint,
older stars contained within the galaxy, suggesting its star formation started at
least 1 billion years ago and possibly as much as 10 billion years ago. The galaxy,
therefore, may have formed at the same time as most other galaxies.
Although the galaxy is not as youthful as was once believed, it is certainly
developmentally challenged and unique in the nearby Universe. Spectroscopic
observations with ground-based telescopes have shown that I Zwicky 18 is almost
exclusively composed of hydrogen and helium, the main ingredients created in the
Big Bang. Heavier elements are forged within the cores of stars and blasted into
space when the stars die. The galaxy’s primordial makeup suggests that its rate of
star formation has been much lower than that of other galaxies of similar age. The
galaxy has been studied with most of NASA’s telescopes, including the Spitzer Space
Telescope, the Chandra X‑ray Observatory, and the Far Ultraviolet Spectroscopic
Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few
stars in the past, and why it is forming so many new stars right now.
The Hubble data also suggest that I Zwicky 18 is 59 million light-years from
Earth, almost 10 million light-years more distant than previously believed. While
this is still in our own backyard by extragalactic standards, the galaxy’s larger-thanexpected distance may explain why astronomers have had difficulty detecting older,
fainter stars within the galaxy until now. In fact, the faint, old stars in I Zwicky 18
are almost at the limit of Hubble’s resolution and sensitivity.
The true age of a galaxy
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Figure 2.1.3. This image of I Zwicky 18
was taken with the Advanced Camera for
Surveys. I Zwicky 18 is classified as a dwarf
irregular galaxy and is much smaller than
our Milky Way Galaxy. The concentrated
bluish-white knots embedded in the heart of
the galaxy are two major starburst regions
where stars are forming at a furious rate.
Astronomers discerned the new distance by observing flashing Cepheid variable
stars in I Zwicky 18. The team determined the observed brightness of three Cepheids
and compared them with the actual brightness predicted by theoretical models. These
models were calculated specifically for I Zwicky 18’s deficiency in heavy elements,
indicating the galaxy’s stars formed before these elements were abundant in the
Universe. This analysis allowed the astronomers to determine the galaxy’s distance.
The Cepheid distance also was validated through another distance indicator: the
observed brightness of the brightest red stars older than 1 billion years.
Cepheid variable stars have been studied for decades and have been instrumental
in the determination of the scale of our Universe. This is the first time, however, that
variable stars with so few heavy elements have been found.
Lego block galaxies in the
early Universe
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The Hubble Space Telescope and Spitzer Space Telescope together have discovered
nine of the smallest, faintest, most compact galaxies ever observed in the distant
Universe. Blazing with the brilliance of millions of stars, each of the newlydiscovered galaxies is a hundred to a thousand times smaller than our Milky Way
Galaxy.
These are among the lowest-mass galaxies ever directly observed in the early
Universe. The conventional model for galaxy evolution predicts that small galaxies
in the early Universe evolved into the massive galaxies of today by coalescing. These
nine Lego-like ‘building block’ galaxies initially detected by Hubble most likely
contributed to the construction of the Universe as we know it.
Astronomers were surprised to find that the galaxies’ estimated masses were
so small, and the Spitzer Space Telescope was called upon to make precise
Figure 2.1.4. In this image of the Hubble
Ultra Deep Field, several objects are
identified as the faintest, most compact
galaxies ever observed in the distant
Universe. They are so far away that we see
them as they looked less than one billion
years after the Big Bang. Blazing with the
brilliance of millions of stars, each of the
newly-discovered galaxies is a hundred to
a thousand times smaller than our Milky
Way Galaxy.
determinations of their masses. The Spitzer observations confirmed that these
galaxies are some of the smallest building blocks of the Universe.
These young galaxies offer important new insights into the Universe’s formative
years, just one billion years after the Big Bang. Hubble detected sapphire-blue stars
residing within the nine pristine galaxies. The youthful stars are just a few million
years old and are in the process of turning Big Bang elements (primarily hydrogen
and helium) into heavier elements. The stars have probably not yet begun to pollute
the surrounding space with elemental products forged within their cores.
While blue light seen by Hubble shows the presence of young stars, it is the
absence of infrared light in the sensitive Spitzer images that was conclusive in
showing that these are truly young galaxies without an earlier generation of stars.
The galaxies were first identified by their prominent and energetic light emission
from glowing hydrogen. Three of the galaxies appear to be slightly disrupted: rather
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than being shaped like rounded blobs, they appear stretched into tadpole-like shapes.
This is a sign that they may be interacting and merging with neighbouring galaxies
to form larger, cohesive structures.
The galaxies were observed in the Hubble Ultra Deep Field (HUDF) with
Hubble’s Advanced Camera for Surveys and the Near Infrared Camera and MultiObject Spectrometer. Observations were also carried out with Spitzer’s Infrared
Array Camera and the European Southern Observatory’s Infrared Spectrometer and
Array Camera.
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