1
00:00:00,000 --> 00:00:03,000
For astronomers, Omega Centauri
has been an outcast

2
00:00:03,000 --> 00:00:06,000
amongst globular clusters
for a long time.

3
00:00:06,000 --> 00:00:09,500
A new result obtained by the
NASA / ESA Hubble Space Telescope

4
00:00:09,500 --> 00:00:13,500
and the Gemini Observatory,
provides a surprising explanation

5
00:00:13,500 --> 00:00:16,500
for Omega Centauri's peculiarities.

6
00:00:35,000 --> 00:00:37,000
This is the Hubblecast.

7
00:00:37,000 --> 00:00:41,000
News and images from the
NASA / ESA Hubble Space Telescope.

8
00:00:41,000 --> 00:00:44,500
Travelling through time and space
with our host, Dr. J

9
00:00:44,500 --> 00:00:48,000
EPISODE 15: Black hole found
in enigmatic Omega Centauri.
a.k.a. Dr. Joe Liske.

10
00:00:48,000 --> 00:00:50,000
Welcome to the Hubblecast.

11
00:00:50,000 --> 00:00:53,000
Today's cosmic guest star
is a very special object.

12
00:00:53,000 --> 00:00:56,000
Omega Centauri has long
been known to be the largest

13
00:00:56,000 --> 00:00:59,000
and brightest globular cluster
visible in the night sky.

14
00:01:00,000 --> 00:01:03,000
A globular cluster is
a nearly spherical group

15
00:01:03,000 --> 00:01:06,000
of tens to hundred
of thousands of old stars

16
00:01:06,000 --> 00:01:09,000
tightly bound together by gravity
and found in the outskirts

17
00:01:09,000 --> 00:01:12,000
of many galaxies
including our own Milky Way.

18
00:01:13,000 --> 00:01:16,000
Beautiful but enigmatic,
Omega Centauri

19
00:01:16,000 --> 00:01:19,000
has always been a bit of a puzzle
to astronomers.

20
00:01:23,000 --> 00:01:26,500
Omega Centauri lies in
the constellation of Centaurus

21
00:01:26,500 --> 00:01:29,500
and is visible from Earth
with the naked eye.

22
00:01:29,500 --> 00:01:33,500
It's one of the favourite celestial objects
for southern hemisphere stargazers,

23
00:01:33,500 --> 00:01:36,000
appearing almost as large
as the full Moon

24
00:01:36,000 --> 00:01:38,000
when seen from a dark site.

25
00:01:38,000 --> 00:01:41,000
Exactly what type of object
Omega Centauri is,

26
00:01:41,000 --> 00:01:43,000
has long been
a contentious topic.

27
00:01:43,500 --> 00:01:46,000
It was first listed
in Ptolemy's catalogue

28
00:01:46,000 --> 00:01:49,000
as a single star nearly
two thousand years ago.

29
00:01:49,000 --> 00:01:53,500
In 1677, Edmond Halley
reported it as a nebulae.

30
00:01:54,000 --> 00:01:57,500
In the 1830s the English astronomer
John Herschel

31
00:01:57,500 --> 00:02:00,500
was the first to recognize it
as a globular cluster,

32
00:02:00,500 --> 00:02:03,500
a classification that
it has kept ever since.

33
00:02:13,000 --> 00:02:15,500
Omega Centauri
has several characteristics

34
00:02:15,500 --> 00:02:18,500
that separate it
from other globular clusters:

35
00:02:18,500 --> 00:02:21,000
compared to a run-of-the-mill globular,

36
00:02:21,000 --> 00:02:24,500
Omega Centauri has
a highly flattened shape,

37
00:02:24,500 --> 00:02:26,500
it rotates faster,

38
00:02:26,500 --> 00:02:29,500
and it includes
several generations of stars,

39
00:02:29,500 --> 00:02:31,500
which is an unusual feature
for globulars

40
00:02:31,500 --> 00:02:35,500
which normally contain only
a single generation of old stars.

41
00:02:35,500 --> 00:02:39,500
Moreover, Omega Centauri
is almost 10 times more massive

42
00:02:39,500 --> 00:02:43,500
than other globular clusters,
almost as massive as a small galaxy.

43
00:02:43,500 --> 00:02:47,500
Now, new images obtained with
the Advanced Camera for Surveys

44
00:02:47,500 --> 00:02:50,000
on-board the NASA / ESA
Hubble Space Telescope

45
00:02:50,000 --> 00:02:54,000
and data obtained with the GMOS
Spectrograph at the Gemini Observatory

46
00:02:54,000 --> 00:02:57,000
show that Omega Centauri
appears to be harbouring

47
00:02:57,000 --> 00:03:01,000
an elusive intermediate-mass
black hole at its centre.

48
00:03:06,500 --> 00:03:08,500
The black hole was discovered
after astronomers

49
00:03:08,500 --> 00:03:13,000
measured the motions and brightnesses
of stars at the centre of Omega Centauri.

50
00:03:13,000 --> 00:03:16,000
They found that these stars
were moving much faster

51
00:03:16,000 --> 00:03:19,500
than expected given their
total number and brightness.

52
00:03:20,000 --> 00:03:23,000
Such behaviour
clearly indicates the existence

53
00:03:23,000 --> 00:03:26,500
of something extraordinarily massive
at the centre of the cluster.

54
00:03:26,500 --> 00:03:29,500
The intense gravitational field
of a black hole

55
00:03:29,500 --> 00:03:32,500
with a mass of 40,000 solar masses

56
00:03:32,500 --> 00:03:35,500
provides just the kick necessary
to explain the measurements.

57
00:03:38,500 --> 00:03:40,500
This is Dr. Eva Noyola

58
00:03:40,500 --> 00:03:44,000
of the Max Planck Institute for
Extraterrestrial Physics in Germany.

59
00:03:44,500 --> 00:03:47,000
So Eva, you were the lead author
on the study.

60
00:03:47,000 --> 00:03:49,000
How did you find this black hole?

61
00:03:49,000 --> 00:03:52,500
The way we found this black hole
was by looking at the Hubble image

62
00:03:52,500 --> 00:03:54,250
and measuring
how many stars are there

63
00:03:54,250 --> 00:03:56,000
at the very centre of this cluster.

64
00:03:56,000 --> 00:03:57,500
And then we take
the GMOS spectrograph

65
00:03:57,500 --> 00:04:00,500
and measure velocities,
the velocities of these stars.

66
00:04:00,500 --> 00:04:02,500
What we find is that the velocities
are much higher

67
00:04:02,500 --> 00:04:04,500
than we would expect
just from the stars.

68
00:04:04,500 --> 00:04:07,500
And that implies that there is
an extra amount of mass, which...

69
00:04:07,500 --> 00:04:09,500
it means that we found a black hole.

70
00:04:09,500 --> 00:04:11,500
And what's so special
about this black hole?

71
00:04:11,500 --> 00:04:13,500
The very special thing
about this black hole

72
00:04:13,500 --> 00:04:15,500
is that is has 40,000
solar masses in size.

73
00:04:16,000 --> 00:04:18,000
So, that means it's a lot bigger

74
00:04:18,000 --> 00:04:21,000
than the stellar-mass black holes
that we find in our Galaxy,

75
00:04:21,000 --> 00:04:23,000
but is much smaller than
the super-massive black holes

76
00:04:23,000 --> 00:04:25,000
that we find
at the centre of galaxies.

77
00:04:25,000 --> 00:04:27,000
So, it means that nature
makes black holes

78
00:04:27,000 --> 00:04:31,000
in a continuous mass range
not just in small and big sizes.

79
00:04:31,000 --> 00:04:33,500
So, what's the big deal?
Why is it so important?

80
00:04:33,500 --> 00:04:35,000
Well, first of all

81
00:04:35,000 --> 00:04:38,000
this is only the second black hole
that we find in a globular cluster,

82
00:04:38,000 --> 00:04:41,000
so that is a very nice
reassuring... thing.

83
00:04:41,000 --> 00:04:43,500
But also we know
that there are seeds

84
00:04:43,500 --> 00:04:47,000
required to grow the super-massive
black holes from scratch.

85
00:04:47,000 --> 00:04:49,000
And if we find many of these,

86
00:04:49,000 --> 00:04:52,000
then that would be a very nice
source for the seeds

87
00:04:52,000 --> 00:04:55,000
to grow the super-massive
black holes.

88
00:04:56,000 --> 00:04:59,000
One implication of this discovery
is that it is very likely

89
00:04:59,000 --> 00:05:02,000
that Omega Centauri
is not a globular cluster at all,

90
00:05:02,000 --> 00:05:04,500
but rather some kind
of dwarf galaxy

91
00:05:04,500 --> 00:05:07,500
that has been stripped of
its outer stars and dark matter

92
00:05:07,500 --> 00:05:11,000
as some scientists have
suspected for a few years.

93
00:05:11,500 --> 00:05:13,500
More than two thousand years

94
00:05:13,500 --> 00:05:16,500
after Omega Centauri was
wrongly classified as a star,

95
00:05:16,500 --> 00:05:19,500
its true nature
is finally coming to light.

96
00:05:19,500 --> 00:05:21,500
But I wonder,

97
00:05:21,500 --> 00:05:24,500
does Omega Centauri have
more surprises in store for us?

98
00:05:25,500 --> 00:05:28,500
This is Dr. J
signing off for the Hubblecast.

99
00:05:28,500 --> 00:05:32,000
Once again, nature has surprised us
beyond our wildest imagination...

100
00:05:33,000 --> 00:05:35,000
Hubblecast is produced
by ESA / Hubble

101
00:05:35,000 --> 00:05:37,000
at the European
Southern Observatory in Germany.

102
00:05:38,000 --> 00:05:40,000
The Hubble mission is a project
of international cooperation

103
00:05:40,000 --> 00:05:42,000
between NASA and
the European Space Agency.