As discussed in the notes, the principal advantage of being in space is that you are
not looking through the Earth's atmosphere. This offers two separate benefits:
- access to wavelengths blocked by the atmosphere
- including γ-rays, X-rays, ultraviolet, most of the infra-red, and submillimetre;
- diffraction-limited resolution
- owing to the lack of atmospheric turbulence. This allows astronomers to study
finer detail in extended objects and to achieve higher precision in position measurements.
There are other more specialised advantages, for example the ability to place spacecraft
in less noisy locations (e.g. WMAP at L2 and Spitzer in Earth-trailing orbit, both to
reduce ambient heat, and therefore minimise cooling needs, for infra-red observations)
and the ability to make very long continuous exposures without worrying about weather or
daylight (e.g. the various Hubble Deep Fields). Mentioning these would improve your
answer, but is not essential since this question asks for a brief explanation.
(The same question could be asked, without the "briefly", for 3 or perhaps even 4 marks -
in this case you certainly would need to mention the minor points as well as the big two.)
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This is a very open-ended question with many "right" answers. The main pitfall is
that your chosen discovery really must rely on space-based instrumentation! (A lot of
things that people think of as "HST discoveries" were actually discovered using
ground-based instruments, althgough they could only be studied in detail by using the
HST.) For this reason, it is wise to play safe and choose things that definitely
cannot be observed from the ground.
The obvious things to choose are the two discoveries explicitly mentioned in the notes:
- X-ray binaries
- Discovered by the Uhuru satellite, which was the first satellite designed
for X-ray observations. Previous X-ray observations from sounding rockets and balloons
had identified "X-ray stars" but had been unable to determine their nature; Uhuru, with
longer exposures, could see periodicity and therefore identify the sources as binaries.
They are now known to be close binary systems in which mass is being transferred from a
normal star to a compact companion (neutron star or black hole). X-ray binaries could not
be detected from the ground because the atmosphere is highly opaque to X-rays: in
many cases the optical counterpart was a known object, but its X-ray behaviour
could not have been determined from the ground.
- Gamma-ray bursts
- Discovered by the Vela satellites, which were actually intended to look for
clandestine nuclear tests. Relative timing between satellites showed that the bursts
being observed were coming from space and not from Earth. Subsequently studied using
astronomical (rather than military) gamma-ray observatories such as CGRO (the Compton
Gamma Ray Observatory). Many gamma-ray bursts do produce optical afterglows, which
can be (and have been) observed by ground-based instruments, but they are faint and
transient and certainly would not have been spotted without the gamma ray signature.
(This is similar to the situation with quasars. The majority of quasars are radio
quiet, and are identified by their optical properties (a blue starlike object with a
large excess of UV radiation and a high redshift). However, there are millions of
catalogued stars, and these objects would not have been spotted as special without the
clue provided by the radio emissions.)
There is no requirement to stick to examples mentioned in the notes. If you know of
other examples from your other courses or from general reading, it is perfectly OK
to use these (for example, the hot X-ray gas in rich clusters of galaxies, which actually
contains nearly all the baryonic matter in the clusters (it outweighs the galaxies by
about a factor of 10!) is completely invisible in the optical and was only detected by
Uhuru; IRAS discovered "IRAS galaxies", anomalously luminous in the infra-red, many
of which turn out to be merging systems giving insight into galaxy evolution and the
origins of active galactic nuclei: like quasars, many of these can be observed in the
optical once you know where to look, but the class would not have been recognised without
the infra-red observations).
You do not have to provide as much detail in answering this question as you would with
a question where the required answer was clearly defined. In an open question like this,
it is recognised that some of the 12 minutes formally allotted to a 3-mark question would
be spent in deciding what class of objects to write about. Basically, one mark comes from
choosing a valid discovery, one for explaining how it was made, and one from discussing why
it could not have been made without space-based instruments.
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