Sky and Telescope - June 2015 - 23

earlier, the Infrared Astronomical Satellite had found
compelling circumstantial evidence for the existence of
dusty disks around young stars, but Hubble was the ﬁrst
to actually image proplyds in exquisite detail.
In fact, Hubble's spectacular views of stellar nurseries
like the Orion Nebula, the Carina Nebula (NGC 3372),
and the Eagle Nebula (M16) provided us with a much
better understanding of the birth of stars and planetary
systems. Using its near-infrared instruments - NICMOS
and, after the last servicing mission in May 2009, Wide
Field Camera 3 - the space telescope has peered into
the dark cores of collapsing, dust-laden clouds. Its sharp
vision revealed energetic bipolar jets from newly formed
protostars, slamming into the surrounding interstellar
medium. Hubble even discovered warps and gaps in
circumstellar disks like the one around Beta Pictoris,
hinting at the presence of planets.
Remember the famous Pillars of Creation found in
the Eagle Nebula in 1995? Hubble found similar pillars
in other stellar nurseries, all of them showing smallscale evidence of being eaten away by the energetic
radiation of nearby clusters of hot, young stars. Over the
past 25 years, we've seen the story of star formation turn
from a sketchy note into a rich novel.
Of course, thanks to ground-based spectrographs
and space-based instruments such as Kepler, we now
know that planetary systems are the rule rather than the
exception. Unsurprisingly, Hubble has also signiﬁcantly
added to our understanding of exoplanets: its transit
observations have revealed the atmospheric constituents
of hot Jupiters, and it even succeeded in directly imaging
a protoplanet-like companion around Fomalhaut.

3. Restless Universe
Some three hundred years ago, Edmund Halley was
the ﬁrst to note that the "ﬁ xed stars" aren't ﬁ xed at all.
Because of their motion through the galaxy, we see them
move across the sky, albeit very slowly.
The proper motion of a distant star is much less
conspicuous than that of a nearby one, for the same
reason that a high-ﬂying jet plane appears to move more
slowly across the sky than a bird that wings right over
your head. Little wonder that we don't notice the proper
motions of stars beyond our local neighborhood.
But Hubble did just that. By comparing images of the
Magellanic Clouds and the Andromeda Galaxy taken
many years apart, astronomers were able to measure the
tiny sideward motion of these galaxies with respect to
the stationary background of distant, point-like quasars.
As a result, we learned that the Magellanic Clouds are
moving so fast that they can't be gravitationally bound to
our Milky Way Galaxy - instead, they are ﬁrst-time visitors (S&T: Oct. 2012, p. 28) - and that the Andromeda
Galaxy really will collide and merge with the Milky Way
a few billion years from now. By virtue of its eagle-eyed

vision, the Hubble Space Telescope has turned our Local
Group into a 3D stage, its main characters moving about
in every direction.
On a smaller scale, Hubble monitored and measured
the expansion of the debris from Supernova 1987A,
showing how it slammed into gas that was blown away
from the star prior to the explosion. It charted the
development of light echoes around variable stars like
V838 Monocerotis and RS Puppis. It revealed changes
and motions in star-forming regions and in jets from
the cores of distant galaxies. Over the past decades, our
universe has become ever more dynamic.

4. Galactic Secrets
Pick up a popular astronomy book from the pre-Hubble
era, and the chapter on galaxies is probably pretty speculative. Yes, they're out there in vast numbers, grouped
together in clusters and superclusters, and they come in
a variety of shapes and types: majestic spirals and barred
spirals, puny irregular dwarfs and giant ellipticals. But
back then, little was known about galactic evolution,
quasars were still rather mysterious, and astronomers
were not yet conﬁdent that supermassive black holes hid
in the cores of most galaxies.
Since it launched, Hubble has observed thousands of
galaxies, including many neighbors of our own Milky
Way out to a few tens of millions of light-years. Not only
did astronomers study individual nebulae, clusters, and
giant stars in other galaxies, they also measured the
bulk rotation in their cores. Over the years, it became
clear that almost every galaxy harbors a supermassive
black hole in its center. What's more, the space telescope
helped reveal a tight correlation between the velocities
of stars in a galaxy's bulge and the mass of the central
black hole deep inside that bulge - evidence for "tandem" evolution of black holes and galaxies' stellar mass.
Hubble's high-resolution images of more remote galaxies, whose light took billions of years to reach Earth, also
revealed galactic interactions, collisions, and mergers in
the early universe. Wrecked spiral arms, warped disks,
tidal tails - these cosmic traﬃc accidents conﬁrmed
the growing conviction that gravitational encounters
explained some galaxies' unusual shapes. Astronomers
also combined Hubble's observations with those from
other instruments to ﬁnd the infant cores of today's giant
elliptical galaxies, which grew largely via mergers.
As for quasars: in its early years, Hubble conﬁrmed
that they are the active, star-like cores of extremely
distant galaxies, powered by supermassive black holes.
Astronomers discovered that powerful quasar winds and
jets may even inhibit the inﬂow of gas into the galaxies,
thus stalling large-scale star formation. Today, the space
telescope's sensitive spectrographs also regularly use
quasar light to study the intergalactic medium - one of
the original Hubble Key Projects.
Sk yandTelescope.com June 2015

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Sky and Telescope - June 2015

Table of Contents for the Digital Edition of Sky and Telescope - June 2015