Sky & Telescope - April 2019 - 20

Galactic Sunset

p GALAXY LENS Left: NGC 6861 in the constellation Telescopium is a lenticular galaxy: It has the dust lanes common to spirals but a whitish oval
shape typical of ellipticals. Right: The Andromeda and Milky Way galaxies eclipse in size this third large spiral of the Local Group, the Triangulum
Galaxy (M33). This ultraviolet image shows the hot, young stars that trace the galaxy's arms.

The Galactic Fountain
The disks of spiral galaxies also lose gas. Stellar winds and
supernova explosions spew gas into the halo. The Milky Way
catapults roughly 10 solar masses out of its disk each year.
That might not sound like the smartest thing for a gas-guzzling galaxy like ours to do, but the material rains back down
onto the disk and brings some halo gas with it, says Filippo
Fraternali (University of Groningen, The Netherlands). The
ejected gas, though hotter than the Sun's surface, is much
cooler than the million-degree halo gas. So the cool gas chills
some of the hot, which condenses onto the cool, just as water
in warm indoor air condenses onto cold glass windows during
winter. According to Fraternali, this process boosts the mass
of gas falling back into the disk by 10% to 20%.
"This 10% to 20% is enough," Fraternali says. The infall
supplies 1 or 2 solar masses per year of extra gas, he says. "This
is exactly the right amount that you need to feed the star for20

A PR I L 2 019 * SK Y & TELESCOPE

mation." As long as a spiral galaxy keeps making stars, he says,
its disk should be able to retrieve new gas from the halo.

Twilight for Spirals
Thus, spiral galaxies do have some tricks up their arms. But
no one knows whether the proposed solutions will really supply enough gas to maintain these magniﬁcent galaxies. How
much longer will they survive? "I think most spiral galaxies
will be running out of gas within 2 to 3 billion years," Fox
says. "The exception might be a case like the Milky Way. We
are fortunate that we have this nearby fuel reserve in our
backyard." If most of the Magellanic Stream gas joins the
Milky Way's disk, he says, our galaxy could retain its spiral
shape for another 5 or 6 billion years, long enough to tide us
over until our encounter with Andromeda.
"Someday the Milky Way will form its last star," says Robert Benjamin (University of Wisconsin, Whitewater). "Given
all the uncertainties we have in our understanding of galaxy
evolution and galaxy environment, we have no ﬁrm way of
predicting that date." He guesses it could be any time from 5
to 15 billion years from now.
Because spiral galaxies have thrived for so long, Elmegreen
suspects that most will manage to acquire enough gas to sus-

LEN TICUL A R G A L A X Y: ESA / HUBBLE / N ASA; M33: N ASA / JPL- CA LTECH;
SPIR A L DR AWINGS: MIA K IE V Y / DIGITA LVISION V ECTORS / G E T T Y IM AG ES

emits X-rays and absorbs radiation from background quasars.
Hot, diffuse gas can't make stars, however. Unless this gas
cools and sinks into the disks, it will do the spirals no good,
in which case they'll resemble thirsty sailors at sea who see
water all around them that they can't drink.

Sky & Telescope - April 2019

Table of Contents for the Digital Edition of Sky & Telescope - April 2019