Sky & Telescope - August 2024 - 37

Whatever the cause, when the gas is squeezed, gravitational
collapse can begin. The gas will fragment into smaller
and smaller chunks. Eventually, these chunks develop into a
cluster of protostars, which continue to grow as gas pours in.
Researchers starting in the 1970s pointed out that, once a
growing star reaches 20 solar masses, the outward pressure
from the star's own photons should clear away the remaining
infalling material. But if this were the whole story, then stars
would not be able to grow beyond 20 solar masses.
Subsequent work has shown that the remedy lies in the
star's accretion disk. This disk encircles the forming star and
channels material down onto it; feeding from the disk is how
a protostar grows. The disk also funnels the star's outpouring
radiation in certain directions, limiting its ability to erode the
infalling gas.
The disk helps determine how big the star will be. While
neat accretion disks typically lead to modest-size stars, larger
stars form more chaotically, with streams of infalling material
acting like conveyor belts, dumping gas onto the forming
star in sudden bursts.
Although theoretical models aren't perfect, they do allow
us to tweak variables to see how environment affects stellar
mass. Models also allow us to envision the conditions in a
location that is very difficult to observe - the early universe
- to see if the first stars could have been more massive than
their modern-day counterparts.
Long Ago and Far Away
Donghui Jeong (Penn State University) and James Gurian
(Perimeter Institute, Canada) use computer modeling to
understand the characteristics of the first stars, which formed
Star Formation, the Early Years
more than 13 billion years ago. Their models are missing one
thing that stellar models of the nearby universe include: a
rich array of elements.
All elements heavier than hydrogen, helium, and lithium
were formed thanks to stars - either in stellar fusion, in
stars' deaths, the collisions of stellar remnants, or by the later
breakup of the atoms that stars make. With no stars before
them to make heavy elements, such as carbon and iron, the
first generation of stars should have been pristine. These stars
are dubbed Population III stars.
Heavy elements can have a major impact on a star's birth
and evolution. Jeong explains that two of the most important
ingredients in determining the final mass of a Population III
star are rotation - which can determine how much gas falls
onto the protostar - and how efficiently that infalling gas
can cool. Heavy elements affect the latter.
It may seem counterintuitive, but gas must first cool
before it forms a star: Gas that is too hot will resist gravity's
inward tug. As gas radiates away this energy, it can collapse
and become gravitationally bound. This cooling occurs due to
collisions within the gas cloud. Collisions can excite electrons
within an atom or a molecule, or the collision's energy can
be stored in the atom or molecule's rotational or vibrational
modes. When the atom or molecule releases this energy
again, it may emit a photon, which carries away some of the
energy within the gas. Thus, the gas cools.
Heavy elements can host electrons at a wider variety of
energy levels than hydrogen or helium can and have more
available rotational and vibrational modes. If there are heavy
elements present in the gas, then there are more ways for
atoms and molecules to lose energy, and the gas can cool
MOLECULAR CLOUD Stars are born in
clouds of dust and molecular gas.
DENSE CORE Dense regions in the cloud
collapse due to their own gravity. As they collapse,
they rotate.
PROTOSTAR The star forms inside the
collapsing core, fed from a disk of gas
(box). The star ejects material as outflows
from its poles, and the disk funnels the
star's radiation in these directions, too,
protecting the disk from erosion.
sk yand tele scope .o r g * AUGUST 2024 37
GREGG DINDERMAN AND BEATRIZ INGLESSIS / S&T, SOURCE: K. PEEK
https://skyandtelescope.org/
Sky & Telescope - August 2024

Table of Contents for the Digital Edition of Sky & Telescope - August 2024
