The initial mass function

The Initial Mass Function
The elliptical galaxy NGC 1600, approximately 200 million light-years away – shown in the center of the Hubble image and highlighted in the box. Astronomers have concluded from the study of this and similar galaxies that the relative populations of stars of different masses in a cluster of stars (the IMF) is influenced by the distribution of velocities in the cluster. Credit: NASA / ESA / Digital Sky Survey 2

The gas and dust in giant molecular clouds gradually come together under the influence of gravity to form stars. Precisely how this occurs, however, is incompletely understood. The mass of a star, for example, is by far the most important factor constraining its future evolution, but astronomers do not clearly understand what determines the exact mass of a newly forming star. One aspect of this problem is simply knowing how many stars of each size there are, that is, knowing the distribution of stellar masses in a large cluster of stars. The initial mass function (IMF) describes this distribution, and is currently based on an average from observations of stars in our Milky Way.

The observed IMF has relatively few massive (i.e., ones more massive than the sun). Sun-sized stars are comparatively abundant. Stars somewhat smaller than the sun are even more common, but then stars of decreasing mass (down to one-tenth of the sun's mass or even less) decrease in numbers. The precise statistics for low mass stars are somewhat uncertain because they are faint and hard to detect. The theoretical basis for the IMF is also being debated, as is whether the IMF of the Milky Way is representative of the IMF elsewhere in the universe. The relative abundance of elements (the "metallicity') in the collapsing cloud, for example, has been suggested as one way to modify the IMF. The idea of a universal IMF, however, has been a cornerstone of stellar theory for decades, but recently there has been considerable effort to test and challenge this assumption, made possible in part by sensitive instruments capable of measuring stars that are smaller and/or fainter. Since stars of different masses have atmospheres showing different spectral features, spectroscopy of a distant cluster whose individual stars cannot be resolved can nevertheless reveal the proportions of stars of different masses within it from the proportions of these features.

CfA astronomer Charlie Conroy and four colleagues are conducting a study of the IMF with the Keck telescope and its spectrometer. They do find some variations in the IMF and, contrary to some expectations, they conclude that metallicity is not the sole driver of these variations. Instead, they conclude that the velocities of the material in the star clusters seems to be a key factor. The result, which now will be followed up with more measurements, is important because it suggests a different theoretical framework is needed to explain the origin of the IMF.


Explore further

An abundance of small stars

More information: Alexa Villaume et al. Initial Mass Function Variability (or Not) among Low-velocity Dispersion, Compact Stellar Systems, The Astrophysical Journal (2017). DOI: 10.3847/2041-8213/aa970f
Journal information: Astrophysical Journal

Citation: The initial mass function (2017, December 11) retrieved 21 May 2019 from https://phys.org/news/2017-12-mass-function.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
38 shares

Feedback to editors

User comments

Dec 11, 2017
From the first sentence, I thought the key factor had to be the velocity of the gas; the stuff about metallicity sounds wrong on the face of it.

Imagine you have an evenly distributed gas cloud that's billions of miles across; almost everywhere, the pull of gravity toward the edges is the same as toward the center. To collapse it, you need to set the gas swirling, so you get clumps that can exert more gravitational pull on the material around them. (Some other star forming or exploding provides the energy; yes that is the chicken-and-egg issue, I don't know the ultimate source, but neither do you.)

Now imagine you have the same cloud, but it's more metallic. How does that change anything? It doesn't.

For whoever goes on about the "surface is like water," imagine a toilet bowl. It just sits there in equilibrium. Doesn't matter what's in the bowl until you flush. Then it swirls and pulls everything downward toward the center. And that's your star formation.

Dec 11, 2017
snw, love your toilet bowl analogy!

And, to a point, I can agree that turbulence plays a part in contraction of lite-elemental dust and gasses. But how much is still anybody's guess. And just because it was accurate with this cloud? Does not mean it will be the same with that cloud over there.

I think the writers of this article are speculating that clouds of dust and gasses containing heavier-elements would have more gravitational attraction due to their heavier mass.

Resulting in a higher rate of infall. Enough to form proto-stars and planetesimals.

Old Motivational Poster
Shows a little boy standing up from the potty. As his Mother, with a roll of TP in one hand. Bends over to stop him pulling up his shorts. Wagging a finger of her other hand and saying. "No, no Son. The job isn't finished until the paperwork is done!"

Truisms we all have to live by. Even in Outer Space!

Dec 11, 2017
And, to a point, I can agree that turbulence plays a part in contraction of lite-elemental dust and gasses. But how much is still anybody's guess. And just because it was accurate with this cloud? Does not mean it will be the same with that cloud over there.

I think the writers of this article are speculating that clouds of dust and gasses containing heavier-elements would have more gravitational attraction due to their heavier mass.

I guess you could get a higher-mass star from a smaller cloud, though.

Resulting in a higher rate of infall. Enough to form proto-stars and planetesimals.


If the cloud is evenly dispersed, then there won't be more gravitational attraction toward the center. Unless all the heavy elements are in the middle. But that's not evenly dispersed. And if the heavy elements are in the middle, you'd have to explain why, so you don't actually end up with an explanation. Metallicity just doesn't explain things by itself.

Dec 11, 2017
In astronomy metallicity refers to the proportion of any elements other than hydrogen and helium.
The key is not the density, but that a lot of these elements are better at radiating infrared, which helps a gas cloud cool, and thus makes it easier for a gas cloud to contract.

Low-metallicity gas clouds can form very high mass stars.

Dec 12, 2017
How are the stars and planets formed?
When Aether, filled with an infinite universe, forms a quark gluon plasma, magnetors are formed, behind them and quasars, and dual stars, after neutron stars and supernovae. When the supernova explodes, gases are formed, in the first place hydrogen, helium and so on. Gravity, as Aether's relation to quarks and magnetism as Aether's relation to gluons, causes the mass compaction of the clouds of gas until the celestial body is obtained. The size depends on the mass of the supernova and the ratio of the aether and gravitation at certain formation sites. From one supernova, depending on the surrounding celestial bodies and their size.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more