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QI

DO RED DWARF STARS GO

THROUGH THE SAME LIFE CYCLE

AS STARS LIKE THE SUN, OR IS THEIR

PROCESS DIFFERENT?
Steve White
Burnaby, British Columbia

AI

While both red dwarfs and stars like the Sun

(solar-type stars) begin and end their lives simi-

larly, their paths diverge during the intermediate stages.

A few million years after their birth, a star’s central

core reaches a temperature high enough to support

sustained nuclear reactions, generating energy by fusing

hydrogen into helium. During this phase of evolution,

red dwarfs and solar-type stars behave relatively simi-

larly. One major difference is that red dwarfs are much

dimmer and hoard their nuclear fuel over longer spans

of time. So, while a solar-type star can burn hydrogen

for only about 10 billion years, some red dwarfs can do

so for trillions of years.

But, as the stars grow older and eventually exhaust

their hydrogen fuel, changes between their life cycles

begin to show. At this stage, solar-type stars grow into

red giants, becoming much brighter, larger, and some-

what cooler (hence their red appearance). In contrast,

red dwarfs remain small in radius but become slightly

brighter and hotter (appearing blue). Another key dif-

ference is that stars like the Sun can successfully burn

helium into carbon and oxygen, whereas small stars

cannot and are left with a largely helium composition

after exhausting their hydrogen supply.

Despite these differences, the endgame is similar for

both types of stars. After all of the possible nuclear

reactions have been carried out, both types of stars end

their lives as white dwarfs. The solar-type stars blow off

much of their original mass and are composed primar-

ily of carbon and oxygen as they condense into white

dwarfs. Red dwarfs retain most of their original mass

and become white dwarfs composed primarily of

helium. Regardless of their composition, these stellar

remnants no longer actively generate energy via nuclear

processing. Instead, they shine with the residual energy

lef t over from their previous epochs of stardom.

Fred Adams

Professor of Physics, University of Michigan, Ann Arbor, Michigan

QI

IS THE DARK MATTER SHAPE OF

THE MILKY WAY THE SAME AS THE

LUMINOUS MATTER SHAPE?
Jack Kessler
Walnut Creek, California

AI

Determining where dark matter lies is a dif-

ficult task, since we cannot observe it directly.

Astronomers can only infer its presence from the

motions of stars in the galaxy, which makes it diffi-

cult to determine a precise shape. Comparatively, it’s

pretty easy to figure out how stars within the Milky

Way Galaxy are distributed (i.e., most are in a disk),

since we can observe them.

One of the most accurate ways to determine the dark

matter shape of a galaxy is to trace the motion of satel-

lite galaxies as they orbit their host galaxy. This is easi-

est to do for satellite galaxies that have been torn apart

by the host galaxy. One of the most famous for the

Milky Way is the Sagittarius Dwarf Galaxy. Some

70,000 light-years away, Sagittarius’ bundle of stars has

been shredded and stretched into a thin stream of stars

around the Milky Way. While the orbits of other, less-

disrupted galaxies and star clusters may seem ideal for

measuring the distribution of dark matter in the Milky

Way, their timetables are much too slow for human

lifetimes. But streams like Sagittarius have their recent

orbits drawn into their shape.

The latest models of Sagittarius and other streams

suggest that the Milky Way’s dark halo has a shape that

changes depending on the distance from the center of

the galaxy. In the inner regions, close to the disk, the

dark matter halo appears to be a f lattened sphere, like

a football, with the long axis pointing in the direction

of the disk. But as one moves outwards, the halo f lips

so the football is balanced on one of its points as the

ga la x y swirls around it.

So, in summary, the answer to the question is: No,

the dark matter and the luminous matter have rather

different shapes.

Amina Helmi

Astronomer, University of Groningen, Groningen, Netherlands

Streams of stars

ripped from a

companion galaxy

circle the Milky Way in

this artist’s concept.

By looking at similar

streams from the

Sagittarius Dwarf

Galaxy, astronomers

were able to piece

together our galaxy’s

dark matter shape.

NASA/JPL-CALTECH/R. HURT (SSC/

CALTECH)