Sky & Telescope - March 2020 - 34

GALACTIC VANDAL

by Shannon Hall

Data from the Gaia spacecraft are unveiling the
Milky Way's tumultuous past, shedding light on
dark matter and our galaxy's violent future.

A

mina Helmi couldn't believe her eyes. It was a little
after noon on a sunny spring day in the Netherlands,
and she was staring at a plot that she had been waiting to create for 20 years. To a bystander, it wouldn't look
like much: a large blob of dots in the shape of a triangle. But
Helmi knew that it had a big story to tell.
Those dots were stars. And within half an hour, she and
her colleagues recognized the pattern of stars as a scar from
a truly colossal collision - one that disfigured our galaxy
roughly 10 billion years ago. That's when the Milky Way
swallowed a smaller companion in a crash that massively
altered the galactic disk and scattered stars far and wide.

The encounter was the latest in a number of monumental mergers that slowly bulked up our galaxy over time. But
then the Milky Way grew eerily quiet. For the past 10 billion
years, only small events have rocked it, allowing it to live
a relatively peaceful existence. But that is now changing.
Today, some small dwarf galaxy is likely wreaking havoc on
our home, sending ripples and waves throughout the galactic
disk. Soon, two other galaxies will come hurtling in.
That is the picture that has emerged over the last three
years thanks to Gaia, a European mission that is charting the
heavens in unprecedented detail. Although astronomers have
long suspected that the Milky Way devoured a number of
unlucky small neighbors, uncovering evidence has not been
easy. But with Gaia's data, astronomers can now hunt down
alien interlopers - stars born within smaller galaxies that
were later cannibalized by our monstrous galaxy - and thus
piece together the Milky Way's violent biography.

Torrents

of the
ANCIENT COLLISION Some 10 billion years
ago, the Milky Way subsumed a smaller
galaxy, whose stars now move through the
Milky Way on elongated, retrograde trajectories (yellow arrows from simulated event,
in galaxy smeared to evoke ours).

34

MARCH 2 02 0 * SK Y & TELESCOPE



Sky & Telescope - March 2020

Table of Contents for the Digital Edition of Sky & Telescope - March 2020

Contents
Sky & Telescope - March 2020 - Cover1
Sky & Telescope - March 2020 - Cover2
Sky & Telescope - March 2020 - 1
Sky & Telescope - March 2020 - Contents
Sky & Telescope - March 2020 - 3
Sky & Telescope - March 2020 - 4
Sky & Telescope - March 2020 - 5
Sky & Telescope - March 2020 - 6
Sky & Telescope - March 2020 - 7
Sky & Telescope - March 2020 - 8
Sky & Telescope - March 2020 - 9
Sky & Telescope - March 2020 - 10
Sky & Telescope - March 2020 - 11
Sky & Telescope - March 2020 - 12
Sky & Telescope - March 2020 - 13
Sky & Telescope - March 2020 - 14
Sky & Telescope - March 2020 - 15
Sky & Telescope - March 2020 - 16
Sky & Telescope - March 2020 - 17
Sky & Telescope - March 2020 - 18
Sky & Telescope - March 2020 - 19
Sky & Telescope - March 2020 - 20
Sky & Telescope - March 2020 - 21
Sky & Telescope - March 2020 - 22
Sky & Telescope - March 2020 - 23
Sky & Telescope - March 2020 - 24
Sky & Telescope - March 2020 - 25
Sky & Telescope - March 2020 - 26
Sky & Telescope - March 2020 - 27
Sky & Telescope - March 2020 - 28
Sky & Telescope - March 2020 - 29
Sky & Telescope - March 2020 - 30
Sky & Telescope - March 2020 - 31
Sky & Telescope - March 2020 - 32
Sky & Telescope - March 2020 - 33
Sky & Telescope - March 2020 - 34
Sky & Telescope - March 2020 - 35
Sky & Telescope - March 2020 - 36
Sky & Telescope - March 2020 - 37
Sky & Telescope - March 2020 - 38
Sky & Telescope - March 2020 - 39
Sky & Telescope - March 2020 - 40
Sky & Telescope - March 2020 - 41
Sky & Telescope - March 2020 - 42
Sky & Telescope - March 2020 - 43
Sky & Telescope - March 2020 - 44
Sky & Telescope - March 2020 - 45
Sky & Telescope - March 2020 - 46
Sky & Telescope - March 2020 - 47
Sky & Telescope - March 2020 - 48
Sky & Telescope - March 2020 - 49
Sky & Telescope - March 2020 - 50
Sky & Telescope - March 2020 - 51
Sky & Telescope - March 2020 - 52
Sky & Telescope - March 2020 - 53
Sky & Telescope - March 2020 - 54
Sky & Telescope - March 2020 - 55
Sky & Telescope - March 2020 - 56
Sky & Telescope - March 2020 - 57
Sky & Telescope - March 2020 - 58
Sky & Telescope - March 2020 - 59
Sky & Telescope - March 2020 - 60
Sky & Telescope - March 2020 - 61
Sky & Telescope - March 2020 - 62
Sky & Telescope - March 2020 - 63
Sky & Telescope - March 2020 - 64
Sky & Telescope - March 2020 - 65
Sky & Telescope - March 2020 - 66
Sky & Telescope - March 2020 - 67
Sky & Telescope - March 2020 - 68
Sky & Telescope - March 2020 - 69
Sky & Telescope - March 2020 - 70
Sky & Telescope - March 2020 - 71
Sky & Telescope - March 2020 - 72
Sky & Telescope - March 2020 - 73
Sky & Telescope - March 2020 - 74
Sky & Telescope - March 2020 - 75
Sky & Telescope - March 2020 - 76
Sky & Telescope - March 2020 - 77
Sky & Telescope - March 2020 - 78
Sky & Telescope - March 2020 - 79
Sky & Telescope - March 2020 - 80
Sky & Telescope - March 2020 - 81
Sky & Telescope - March 2020 - 82
Sky & Telescope - March 2020 - 83
Sky & Telescope - March 2020 - 84
Sky & Telescope - March 2020 - Cover3
Sky & Telescope - March 2020 - Cover4
https://www.nxtbook.com/nxtbooks/aas/st_202406
https://www.nxtbook.com/nxtbooks/aas/st_202405
https://www.nxtbook.com/nxtbooks/aas/st_202404_qr
https://www.nxtbook.com/nxtbooks/aas/st_202404
https://www.nxtbook.com/nxtbooks/aas/st_202403
https://www.nxtbook.com/nxtbooks/aas/st_202402
https://www.nxtbook.com/nxtbooks/aas/st_202401_aus
https://www.nxtbook.com/nxtbooks/aas/st_202401
https://www.nxtbook.com/nxtbooks/aas/st_202312_aus
https://www.nxtbook.com/nxtbooks/aas/st_202312
https://www.nxtbook.com/nxtbooks/aas/st_202311
https://www.nxtbook.com/nxtbooks/aas/st_202310
https://www.nxtbook.com/nxtbooks/aas/st_202309
https://www.nxtbook.com/nxtbooks/aas/st_202308
https://www.nxtbook.com/nxtbooks/aas/st_202307
https://www.nxtbook.com/nxtbooks/aas/st_202306
https://www.nxtbook.com/nxtbooks/aas/st_202305
https://www.nxtbook.com/nxtbooks/aas/st_202304
https://www.nxtbook.com/nxtbooks/aas/st_202304_qr
https://www.nxtbook.com/nxtbooks/aas/st_202303
https://www.nxtbook.com/nxtbooks/aas/st_202302
https://www.nxtbook.com/nxtbooks/aas/st_202301
https://www.nxtbook.com/nxtbooks/aas/st_202212
https://www.nxtbook.com/nxtbooks/aas/st_202211
https://www.nxtbook.com/nxtbooks/aas/st_202210
https://www.nxtbook.com/nxtbooks/aas/st_202209
https://www.nxtbook.com/nxtbooks/aas/st_202208
https://www.nxtbook.com/nxtbooks/aas/st_202207
https://www.nxtbook.com/nxtbooks/aas/st_202206
https://www.nxtbook.com/nxtbooks/aas/st_202205
https://www.nxtbook.com/nxtbooks/aas/st_202204
https://www.nxtbook.com/nxtbooks/aas/st_202203
https://www.nxtbook.com/nxtbooks/aas/st_202202
https://www.nxtbook.com/nxtbooks/aas/st_202201
https://www.nxtbook.com/nxtbooks/aas/st_202112
https://www.nxtbook.com/nxtbooks/aas/st_202111
https://www.nxtbook.com/nxtbooks/aas/st_202110
https://www.nxtbook.com/nxtbooks/aas/st_202109
https://www.nxtbook.com/nxtbooks/aas/st_202108
https://www.nxtbook.com/nxtbooks/aas/st_202107
https://www.nxtbook.com/nxtbooks/aas/st_202106
https://www.nxtbook.com/nxtbooks/aas/st_202105
https://www.nxtbook.com/nxtbooks/aas/st_202104
https://www.nxtbook.com/nxtbooks/aas/st_202103
https://www.nxtbook.com/nxtbooks/aas/st_202102
https://www.nxtbook.com/nxtbooks/aas/st_202101
https://www.nxtbook.com/nxtbooks/aas/skywatch_2021
https://www.nxtbook.com/nxtbooks/aas/st_202012
https://www.nxtbook.com/nxtbooks/aas/st_202011
https://www.nxtbook.com/nxtbooks/aas/st_202010
https://www.nxtbook.com/nxtbooks/aas/st_202009
https://www.nxtbook.com/nxtbooks/aas/st_202008
https://www.nxtbook.com/nxtbooks/aas/st_202007
https://www.nxtbook.com/nxtbooks/aas/st_202006
https://www.nxtbook.com/nxtbooks/aas/st_202005
https://www.nxtbook.com/nxtbooks/aas/st_202004
https://www.nxtbook.com/nxtbooks/aas/st_202003
https://www.nxtbook.com/nxtbooks/aas/st_202002
https://www.nxtbook.com/nxtbooks/aas/st_202001
https://www.nxtbook.com/nxtbooks/aas/st_201912
https://www.nxtbook.com/nxtbooks/aas/st_201911
https://www.nxtbook.com/nxtbooks/aas/st_201910
https://www.nxtbook.com/nxtbooks/aas/st_201909
https://www.nxtbook.com/nxtbooks/aas/st_201908
https://www.nxtbook.com/nxtbooks/aas/st_201907
https://www.nxtbook.com/nxtbooks/aas/st_201906
https://www.nxtbook.com/nxtbooks/aas/st_201905
https://www.nxtbook.com/nxtbooks/aas/st_201904
https://www.nxtbook.com/nxtbooks/aas/st_201903
https://www.nxtbook.com/nxtbooks/aas/st_201902
https://www.nxtbook.com/nxtbooks/aas/st_201901
https://www.nxtbook.com/nxtbooks/aas/st_201812
https://www.nxtbook.com/nxtbooks/aas/st_201811
https://www.nxtbook.com/nxtbooks/aas/st_201810
https://www.nxtbook.com/nxtbooks/aas/st_201809
https://www.nxtbook.com/nxtbooks/aas/st_201808
https://www.nxtbook.com/nxtbooks/aas/st_201807
https://www.nxtbook.com/nxtbooks/aas/st_201806
https://www.nxtbook.com/nxtbooks/aas/st_201805
https://www.nxtbook.com/nxtbooks/aas/st_201804
https://www.nxtbook.com/nxtbooks/aas/st_201803
https://www.nxtbook.com/nxtbooks/aas/st_201802
https://www.nxtbook.com/nxtbooks/aas/st_201801
https://www.nxtbook.com/nxtbooks/aas/st_201712
https://www.nxtbook.com/nxtbooks/aas/st_201711
https://www.nxtbook.com/nxtbooks/aas/st_201710
https://www.nxtbook.com/nxtbooks/aas/st_201709
https://www.nxtbook.com/nxtbooks/aas/st_201708
https://www.nxtbook.com/nxtbooks/aas/st_201707
https://www.nxtbook.com/nxtbooks/aas/st_201706
https://www.nxtbook.com/nxtbooks/aas/st_201705
https://www.nxtbook.com/nxtbooks/aas/st_201704
https://www.nxtbook.com/nxtbooks/aas/st_201703
https://www.nxtbook.com/nxtbooks/aas/st_201702
https://www.nxtbook.com/nxtbooks/aas/st_201701
https://www.nxtbook.com/nxtbooks/aas/st_201612
https://www.nxtbook.com/nxtbooks/aas/st_201611
https://www.nxtbook.com/nxtbooks/aas/st_201610
https://www.nxtbook.com/nxtbooks/aas/st_201609
https://www.nxtbook.com/nxtbooks/aas/st_201608
https://www.nxtbook.com/nxtbooks/aas/st_201607
https://www.nxtbook.com/nxtbooks/aas/st_201606
https://www.nxtbook.com/nxtbooks/aas/st_201605
https://www.nxtbook.com/nxtbooks/aas/st_201604
https://www.nxtbook.com/nxtbooks/aas/st_201603
https://www.nxtbook.com/nxtbooks/aas/st_201602
https://www.nxtbook.com/nxtbooks/aas/st_201601
https://www.nxtbook.com/nxtbooks/aas/st_201512
https://www.nxtbook.com/nxtbooks/aas/st_201511
https://www.nxtbook.com/nxtbooks/aas/st_201510
https://www.nxtbook.com/nxtbooks/aas/st_201509
https://www.nxtbook.com/nxtbooks/aas/st_201508
https://www.nxtbook.com/nxtbooks/aas/st_201507
https://www.nxtbook.com/nxtbooks/aas/st_201506
https://www.nxtbook.com/nxtbooks/aas/st_201505
https://www.nxtbook.com/nxtbooks/aas/st_201504
https://www.nxtbook.com/nxtbooks/aas/st_201503
https://www.nxtbook.com/nxtbooks/aas/st_201502
https://www.nxtbook.com/nxtbooks/aas/st_201501
https://www.nxtbook.com/nxtbooks/aas/st_201412
https://www.nxtbook.com/nxtbooks/aas/st_201411
https://www.nxtbook.com/nxtbooks/aas/st_201410
https://www.nxtbook.com/nxtbooks/aas/st_201409
https://www.nxtbook.com/nxtbooks/aas/st_201408
https://www.nxtbook.com/nxtbooks/aas/st_201407
https://www.nxtbook.com/nxtbooks/aas/st_201406
https://www.nxtbook.com/nxtbooks/aas/st_mars
https://www.nxtbook.com/nxtbooks/aas/st_201405
https://www.nxtbook.com/nxtbooks/aas/st_201404
https://www.nxtbook.com/nxtbooks/aas/st_201403
https://www.nxtbook.com/nxtbooks/aas/st_201402
https://www.nxtbook.com/nxtbooks/aas/st_201401
https://www.nxtbook.com/nxtbooks/aas/st_201312
https://www.nxtbook.com/nxtbooks/aas/st_201311
https://www.nxtbook.com/nxtbooks/aas/st_201310
https://www.nxtbook.com/nxtbooks/aas/st_201309
https://www.nxtbook.com/nxtbooks/aas/st_201308
https://www.nxtbook.com/nxtbooks/aas/st_201307
https://www.nxtbook.com/nxtbooks/aas/st_201306
https://www.nxtbook.com/nxtbooks/aas/st_201305
https://www.nxtbook.com/nxtbooks/aas/st_201304
https://www.nxtbook.com/nxtbooks/aas/st_201303
https://www.nxtbook.com/nxtbooks/aas/st_201302
https://www.nxtbook.com/nxtbooks/aas/st_201301
https://www.nxtbook.com/nxtbooks/aas/st_201212
https://www.nxtbook.com/nxtbooks/aas/st_201211
https://www.nxtbook.com/nxtbooks/aas/st_201210
https://www.nxtbook.com/nxtbooks/aas/st_201209
https://www.nxtbook.com/nxtbooks/aas/st_201208
https://www.nxtbook.com/nxtbooks/aas/st_201207
https://www.nxtbook.com/nxtbooks/aas/st_201206
https://www.nxtbook.com/nxtbooks/aas/st_201205
https://www.nxtbook.com/nxtbooks/aas/st_201204
https://www.nxtbook.com/nxtbooks/aas/st_201203
https://www.nxtbook.com/nxtbooks/aas/st_201202
https://www.nxtbook.com/nxtbooks/aas/st_201201
https://www.nxtbook.com/nxtbooks/aas/st_201112
https://www.nxtbook.com/nxtbooks/aas/st_201111
https://www.nxtbook.com/nxtbooks/aas/st_201110
https://www.nxtbook.com/nxtbooks/aas/st_201109
https://www.nxtbook.com/nxtbooks/aas/st_201108
https://www.nxtbook.com/nxtbooks/aas/st_201107
https://www.nxtbook.com/nxtbooks/aas/st_201106
https://www.nxtbook.com/nxtbooks/aas/st_201105
https://www.nxtbook.com/nxtbooks/aas/st_201104
https://www.nxtbook.com/nxtbooks/aas/st_201103
https://www.nxtbook.com/nxtbooks/aas/st_201102
https://www.nxtbook.com/nxtbooks/aas/st_201101
https://www.nxtbook.com/nxtbooks/aas/st_201012
https://www.nxtbook.com/nxtbooks/aas/st_201011
https://www.nxtbook.com/nxtbooks/aas/st_201010
https://www.nxtbook.com/nxtbooks/aas/st_201009
https://www.nxtbook.com/nxtbooks/aas/st_201008
https://www.nxtbook.com/nxtbooks/aas/st_201007
https://www.nxtbook.com/nxtbooks/aas/st_201006
https://www.nxtbook.com/nxtbooks/aas/st_201005
https://www.nxtbook.com/nxtbooks/aas/st_201004
https://www.nxtbook.com/nxtbooks/aas/st_201003
https://www.nxtbook.com/nxtbooks/aas/st_201002
https://www.nxtbook.com/nxtbooks/aas/st_201001
https://www.nxtbookmedia.com