Skip to main content
44 votes
Accepted

Are there any stars that orbit perpendicular to the Milky Way's galactic plane?

The Sun and most of the other stars are in the bulging disk of the Milky Way galaxy, but about 1% of the galaxy's stellar mass is in the galactic halo. The halo also includes 50 globular clusters and ...
Connor Garcia's user avatar
  • 16.3k
22 votes
Accepted

Can stars form in the stellar halo?

Stars form from cold gas, not hot. Hot gas consists of fast-moving particles which cannot condense. So stars are born in the densest regions in molecular clouds where the temperature is 10–100 K, and ...
pela's user avatar
  • 38.9k
21 votes

Are there any stars that orbit perpendicular to the Milky Way's galactic plane?

Most stars in the galaxy are in the disc, but there is also a population in the galactic halo, these are in orbits at essentially random inclinations to the disc. There will be some that orbit ...
James K's user avatar
  • 126k
18 votes
Accepted

Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

The friends-of-friends (FOF) algorithm (Huchra & Geller 1982; Press & Davis 1982, Davis et al. 1985) for finding groups of particles can be used for various numerical problems, not just ...
pela's user avatar
  • 38.9k
9 votes

References about neutrinos getting clumped in "structures" in the future?

For realistic values of the small, but non-zero, mass of at least some of the neutrino families, the vast numbers of cosmic background neutrinos (currently at a temperature of around 2.3 K) are not ...
ProfRob's user avatar
  • 156k
8 votes
Accepted

What is the mean separation between a star and its closest neighbor in the Milky Way halo?

I did some rough calculations, and 100 light-years doesn't seem to be a bad guess. If we assume that the average mass of a halo star is $\sim0.3M_{\odot}$, as would be expected for a typical IMF, and ...
HDE 226868's user avatar
  • 37.2k
6 votes
Accepted

The contribution of components like dark matter, stars and gas to the mass of a galaxy

This is a rather complex question, for several reasons. First, galaxies come in many variations, regarding such diverse properties as mass, morphology, and environment. Second, different ...
pela's user avatar
  • 38.9k
5 votes

Why do people choose 0.2 as the value of linking length in the friends-of-friends algorithm?

A quick search brought me to a book by Houjun Mo, et al. Galaxy Formation and Evolution which says For example, the frequently used friends-of-friends (FOF) algorithm defines halos as structures ...
B--rian's user avatar
  • 5,636
4 votes

What object(s) are furthest above/below the galactic plane?

Bochanski et al. (2014; "The Most Distant Stars in the Milky Way") reported the discovery of two stars -- ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0 -- found to be M giants at ...
Peter Erwin's user avatar
  • 17.3k
4 votes
Accepted

How do disk-shaped galaxies then form at the sites of spherical galactic halos? Do the halo stars start orbiting or just sit in place?

The key to understanding this is that dark matter and, effectively, stars are collisionless, whereas gas is collisional. Structure formation As written in the second quote, structure forms in a ...
pela's user avatar
  • 38.9k
3 votes
Accepted

Why is the boundary of friends-of-friends (FOF) halo corresponding to iso-density contour?

If you have enough particles to sample your field, then a given (local) density corresponds to the same mean distance between your particles. The FOF algorithm keeps linking particles to a halo until ...
pela's user avatar
  • 38.9k
3 votes

What object(s) are furthest above/below the galactic plane?

Globular clusters orbit in the Galactic halo, and so are good candidates for being far above the disc. The globular cluster PSO J174.0675-10.8774 is the furthest, 470,000 light years distant, well ...
James K's user avatar
  • 126k
3 votes
Accepted

How to build an adjacency matrix of stars connectivity from their physical kpc distances?

Computationally, a brute force algorithm where you calculate $r_{ij}$ for each pair $(i,j)$ and set $A_{ij}$ if it is smaller than the cut-off distance runs in $O(N^2)$ time. While this was much too ...
Anders Sandberg's user avatar
3 votes
Accepted

Evaluating Potentials using Galpy

For future reference, I believe the issue here was that you didn't instantiate the potential. That is, you probably tried to do ...
Jo Bovy's user avatar
  • 146
2 votes
Accepted

Is it possible to observe Lyman-continuum emission from extragalactic objects?

I think the short answer is "You can't observe $z=0$ LyC leakers, but you can observe $z\sim0$ LyC leakers." No z = 0 leakers… Measuring the Lyman continuum (LyC) — i.e. photons capable of ...
pela's user avatar
  • 38.9k
1 vote
Accepted

What does it mean for cold clouds to be in pressure equilibrium with a diffuse hot medium?

Pressure equilibrium means that the pressure of the hot component and the pressure of the cold component are equal at their interface, so that neither expands nor contracts relative to the other. The ...
Peter Erwin's user avatar
  • 17.3k
1 vote

What is the mean separation between a star and its closest neighbor in the Milky Way halo?

About 300 light years Wolfram has previously informed me that if our planet is as big as a flea (1MM), the sun is as big as a pigeon (11CM), the average distance of milky way halo stars is 25000 KM, ...
bandybabboon's user avatar
  • 4,268

Only top scored, non community-wiki answers of a minimum length are eligible