Star Sorting

H-R DIAGRAM OF
STAR CLASSIFICATION

MAKING A COLOR-MAGNITUDE
DIAGRAM FOR GLOBULAR CLUSTER
OMEGA CENTAURI

THIS IS THE EARLY-RELEASE IMAGE OF OMEGA
CENTAURI, TAKEN BY WFC3/UVIS ON BOARD THE
HUBBLE SPACE TELESCOPE (HST)

A CLOSE UP OF THE
CENTRAL REGION.

THIS IMAGE WAS MADE BY COMBINING
SEPARATE RED, GREEN, AND BLUE IMAGES.

THE RED IMAGE IS FROM FILTER F814W,
WHICH SEES ONLY VERY RED LIGHT.

THE GREEN IMAGE IS FROM FILTER F336W,
WHICH SEES ONLY GREEN LIGHT.

THE BLUE IMAGE IS FROM FILTER F225W,
WHICH SEES ONLY ULTRA-VIOLET LIGHT.

THE COLORS ARE SO EXTREME BECAUSE…

…THE RED STARS HAVE ALMOST NO BLUE LIGHT,
AND THE BLUE STARS HAVE ALMOST NO RED LIGHT.

ASTRONOMERS LIKE TO STUDY THE COLORS OF STARS
IN A QUANTITATIVE WAY.

THEY LIKE TO SORT THE STARS BY COLOR, PUTTING THE
BLUE STARS ON THE LEFT AND THE RED STARS ON THE
RIGHT.

THEY LIKE TO SORT THE
STARS BY COLOR, PUTTING
THE BLUE STARS ON THE LEFT
AND THE RED STARS ON THE
13f

13g

13h

13i

13j

13k

NOTE THAT THERE ARE VERY FEW
EXTREME STARS; MOST STARS ARE
WHITE, MEANING THEY HAVE A
BALANCED SPECTRUM. 14

ASTRONOMERS ALSO LIKE TO
CHARACTERIZE THE STARS IN
TERMS OF BRIGHTNESS.
14

STARS,
PUTTING THE BRIGHT STARS
ON TOP,
AND THE FAINT STARS ON THE 15a

STARS,
PUTTING THE BRIGHT STARS
ON TOP,
AND THE FAINT STARS ON THE 15b

THEY LIKE TO SORT THE STARS,
PUTTING THE BRIGHT STARS ON TOP,
AND THE FAINT STARS ON THE
BOTTOM.

BOTTOM.
15e

BOTTOM.
15f

BOTTOM.
15g

BOTTOM.
15h

BOTTOM.
15i

BOTTOM.
15j

BOTTOM.
15k

THIS IS CALLED A
COLOR-MAGNITUDE DIAGRAM
(CMD).
16

WHEN ASTRONOMERS FIRST
PLOTTED STARS THIS WAY, THEY
REALIZED THAT STARS DON’T FALL
JUST ANYWHERE IN THE DIAGRAM. 17

STARS TEND TO LIE ALONG
A FEW WELL-DEFINED
SEQUENCES.
18

THE UNMISTAKABLE ORDER IN
DIAGRAMS LIKE THIS LED
ASTRONOMERS TO DEVELOP THEORIES
TO EXPLAIN STELLAR EVOLUTION. 19

THE VAST MAJORITY OF STARS
LIE ALONG THE MAIN SEQUENCE
(MS).
20a
MS

STARS DON’T MOVE ALONG THIS SEQUENCE;
RATHER THEY SIT AT THE SAME PLACE FOR
A LONG TIME FUSING THEIR HYDROGEN INTO
HELIUM. 20b
MS

THIS IS A SEQUENCE IN MASS.
STARS AT THE BRIGHT END ARE
MASSIVE, AND THOSE AT THE FAINT,
RED END HAVE VERY LOW MASSES. 20c
MS

THE MORE MASSIVE STARS
CONSUME THEIR HYDROGEN FUEL
MUCH FASTER THAN THE LOWER-
MASS STARS. 20d
MS

WHEN FUEL BECOMES SPARSE IN THE STELLAR
CORE, STARS READJUST THEIR INTERNAL
STRUCTURE AND MOVE RED-WARD ALONG THE
SUB-GIANT BRANCH (SGB). 21
MS
SGB

THEY START TO BURN THE HYDROGEN IN
A SHELL AROUND THE CORE AND BECOME
BIG AND BLOATED AS THEY MOVE UP THE
RED GIANT BRANCH (RGB). 22a
MS
SGB
RGB

AS THE SHELL-BURNING CONTINUES,
MORE AND MORE HELIUM GETS
DEPOSITED ONTO THE CORE.
22b
MS
SGB
RGB

WHEN THE CORE HAS ENOUGH MASS, IT IS
FINALLY ABLE TO IGNITE HELIUM INTO
CARBON.
22c
MS
SGB
RGB

THE STAR READJUSTS ITS STRUCTURE
AGAIN AND FINDS ITSELF ON THE
HORIZONTAL BRANCH (HB).
23a
MS
SGB
RGBHB

THE HELIUM FUEL IS NOT AS
POTENT AS THE HYDROGEN, SO
IT RUNS OUT QUICKLY.
23b
MS
SGB
RGBHB

THAT IS WHY THERE ARE SO FEW
STARS ON THE HB COMPARED TO THE
MS. STARS DO NOT SPEND MUCH
TIME ON THE HB. 23c
MS
SGB
RGBHB

WHEN THE HELIUM IS GONE, THE STAR
HAS NO MORE FUEL. WITH NOTHING LEFT
TO BURN, IT FADES AWAY INTO BLUE
DARKNESS AS A WHITE DWARF (WD). 24
MS
SGB
RGBHB
WD

SINCE A STAR’S
COLOR AND
BRIGHTNESS TELL
US ITS
EVOLUTIONARY
PHASE, WE CAN
EASILY IDENTIFY
STARS BY PHASE
IN THE IMAGE.
25
MS
SGB
RGBHB
WD

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