Sky & Telescope - USA (2020-06)

The First Stars

64 JUNE 2020 • SKY & TELESCOPE

A Faint Hope

While most models of Pop III stars suggest they should have

expired long ago, there remains a lingering uncertainty. Com-

puter simulations in 2014 by Athena Stacy (then at University

of California, Berkeley) and Volker Bromm (University of

Texas, Austin) suggest that not all Pop III stars formed big

and died quickly. Some less massive than the Sun may have

formed in conjunction with a much higher-mass companion.

If so, slow-burning and low-mass Pop III stars might remain

nearby, although they would likely be quite rare and pos-

sibly contaminated by heavy elements from the interstellar

medium or a companion star.

Would current techniques of stellar archeology allow us to

fi nd them? “These stars, if they exist locally, would be freaks

of nature,” says Frebel. “But we cannot yet exclude the possi-

bility of their existence. I think we could recognize these stars

if we measured them.”

Bromm also believes we could recognize Pop III stars. It’s

unlikely such stars could masquerade as low-mass Pop II stars,

he says — the chemical signature of the contamination they

had experienced would be different. Yet he is not optimistic

about fi nding them. “The chances to fi nd these stars are fad-

ing because survey sizes are so large that, if low-mass Pop III

stars existed, we should have found them by now,” he says.

New projects such as the Canada-France-Hawaii Tele-

scope’s Pristine survey and results from larger ground-based

telescopes will likely fi nd even more metal-poor stars with

interesting chemical signatures. The European Space Agency’s

Gaia mission is also helping link the chemistry of these

ancient stars with their motions through the galaxy. Astrono-

mers are already using such data to identify large groups of

stars that are the likely remains of destroyed dwarfs (S&T:

Mar. 2020, p. 34).

Like its earthbound counterpart, stellar archaeology is

a challenging fi eld. But when it comes to the night sky, the

deeper you dig, the more you fi nd.

¢ An erstwhile laser physicist and longtime stargazer with

degrees in astronomy and applied physics, BRIAN VENTRUDO

now observes and writes about stars of all ages from his home

in Calgary, Canada.

Merging neutron stars

Exploding massive stars

Very radioactive isotopes; nothing left from stars

1

H

3

Li

11

Na

19

K

37

Rb

55

Cs

87

Fr

4

Be

12

Mg

20

Ca

38

Sr

56

Ba

21

Sc

39

Y

22

Ti

40

Zr

72

Hf

23

V

41

Nb

73

Ta

24

Cr

42

Mo

74

W

25

Mn

43

Tc

75

Re

26

Fe

44

Ru

76

Os

27

Co

45

Rh

77

Ir

28

Ni

46

Pd

78

Pt

29

Cu

47

Ag

79

Au

30

Zn

48

Cd

80

Hg

31

Ga

49

In

81

Tl

32

Ge

50

Sn

82

Pb

33

As

51

Sb

83

Bi

34

Se

52

Te

84

Po

35

Br

53

I

85

At

36

Kr

54

Xe

57

La

89

Ac

58

Ce

90

Th

59

Pr

91

Pa

60

Nd

92

U

93

Np

94

Pu

61

Pm

62

Sm

63

Eu

64

Gd

65

Tb

66

Dy

67

Ho

68

Er

69

Tm

70

Yb

71

Lu

86

Rn

5

B

13

Al

6

C

14

Si

7

N

15

P

8

O

16

S

9

F

17

Cl

2

He

10

Ne

18

Ar

88

Ra

Big Bang

Dying low mass stars

Exploding white dwarfs

Cosmic ray fission

pRETICULUM II Ultra-faint dwarf galaxies (UFDs) contain so few stars

that they’re diffi cult to spot. Foreground stars mask the UFD Reticulum

II in a Dark Energy Camera image (left, with brightest stars blacked out

by bars). Only after blacking out all other visible matter can we see the

stars that belong to the tiny galaxy (right, stars’ bubble appearance is a

consequence of the image processing).

qORIGINS OF THE ELEMENTS The solar system’s elements have different cosmic origins, with many of the heaviest made in neutron-star mergers

(dark blue). Those in black are either artifi cially made or unstable on long time scales.

RETICULUM II: FERMILAB / DARK ENERGY SURVEY (2); PERIODIC TABLE: LEAH TISCIONE /

S&T

, SOURCE: JENNIFER JOHNSON /

SCIENCE

2019