11

Radiative energy transport continues. The point is that the radiative flux, which is proportional to $dT/dr$ can be overtaken when the temperature gradient achieves the adiabatic value and convection starts. Once convection is started, it is very efficient and the majority of energy flux will be transported by convection. Details Broadly speaking, radiative ...


10

This is a great series of questions! Such a low mass black hole (BH) could have originated from a few possibilites: 1) a result of stellar evolution (the resulting black hole mass depends fundamentally on the initial mass and metallicity of the stellar progenitor, among other things); 2) a star collapsed into a neutron star which can accrete matter from its ...


10

A black hole of a given mass will probably have arisen from the collapse/supernova of a much more massive star. In particular, stars with an initial mass of less than around 15-20 solar masses are unlikely to leave a black hole remnant at all. Stars of $<8$ solar masses end their lives as white dwarfs and those with $8$ to $\sim 15$ solar masses likely ...


10

$$\frac{dP}{dr} = - \rho g,$$ is the equation of hydrostatic equilibrium, where $\rho$ and $g$ are the local density and gravity, $P$ is pressure and $r$ is the radial coordinate. This can be rewritten as $$\frac{d\rho}{dr} \frac{dP}{d\rho} = -\rho g.$$ Since $\rho$ and $g$ are positive numbers, the pressure gradient is negative. For all types of matter $P = ...


6

Collisional broadening - which includes van der Waals and Stark broadening - is more important in the higher gravity, higher pressure/density atmospheres of dwarf stars (a factor of 100-1000 higher for dwarfs vs giants of the same photospheric temperature). These collisional effects effectively "truncate" radiative emission and absorption processes,...


6

The role of H$_2$ is to allow primordial gas to cool down sufficiently to allow the collapse to start and then to hold the gas as a relatively low temperature as it gets much more dense. The formation of H$_2$ is essential because atomic hydrogen simply has no way of cooling itself below temperatures that are capable of exciting the $n=2$ level and then ...


5

Not as a steady state, for sure. If there happen to be a part less dense than its surroundings it would just float to the surface. This is called buoyancy and the math is basic enough. Stars don't have solid parts and cannot sustain a state different from the hydrostatic near-equilibrium. Then again, some transient event like supernova Ia explosion may ...


5

It seems that the authors are just referring to the accepted model of a pulsar, i.e. a neutron star spinning and emitting beams of radiation at its poles. In that sense, the term is used here no differently than it would be in the context of an isolated pulsar. It's just a very simple way of visualizing why an observer far away appears to see periodic pulses ...


4

OK! So it's night time in the desert. If it is hot or very warm it's summer, even at night. If it is cold it's most likely winter. If it is a pleasant temperature or mild, it's anyone's guess. If you are having difficulty breathing and it is cold, you are on a very high mountain - Andes, Himalayas. If it is overcast and you don't recognize any on the ...


4

Magnesium does not have its own fusion shell inside stars. If you have a look at the Nuclear Binding Energies per nucleon(NBE) of Elements, you will notice one trend: Most of the elements that form a shell have a higher value of NBE locally. Nuclear binding energy is the minimum energy that is required to disassemble the nucleus of an atom into its ...


4

A number of neutron stars in binary systems have measured masses below $1.44 M_\odot$ (e.g. a pulsar of mass $1.251 \pm 0.021 M_\odot$, McKee et al. 2020). I think the current lowest mass contender is $1.174 \pm 0.004 M_\odot$ (Martinez et al. (2015). See the plot below with a pictorial representation of the current state of neutron star mass measurements ...


4

A large fraction of M-dwarfs are highly magnetically active because they have deep convection zones (or may be fully convective) and they have longer spindown timescales than higher mass stars, so remain at higher magnetic activity levels for longer. The magnetic fields are generated by some sort of internal dynamo. The dynamo efficiency appears to increase ...


4

The blueward evolution is a simple consequence of the homology relations for main sequence evolution. If the star is rotating very fast, then this induces strong mixing of the interior, such that the star can be treated as chemically homogeneous. As main sequence hydrogen burning proceeds, the entire star is gradually turned into helium, with a consequent ...


3

Higher mass stars will have shorter lives. Even though they have more fuel for nucleosynthesis, they burn this fuel much quicker than lower mass stars. Generally, you should think of "red-giant" as an evolutionary phase and not a particular type of star. Looking at the Scheller et al. (Aston. Astrophys. Suppl., 96, 269, 1992) data given in one of ...


2

First off, let us try to clarify a few terms: As usual in astrophysics, metal-free star means atomic number $Z \leq 3$, i.e. it only consists of the primordial elements hydrogen, helium, and lithium. Primordial star literally means original star and refers to the first star(s) (generation) formed after the big bang. It is IMHO equivalent to metal-free, and ...


2

As is the case with light traveling through any medium, radio waves traveling through space experience refraction, which reduces their speed. A wave of infinite frequency will experience no refraction, meaning that we can compute how much a given wave will be delayed compared to such an infinite-frequency wave as it moves through outer space. The group ...


2

Below about 300 MHz you can only see the sun's corona as the frequency is too low to penetrate the coronal plasma from further below. In this Wikipedia article you can see images of the sun from 17 GHz down to 80 Mhz. By Patrick McCauley Mccauley.pi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=99231226 Photos and a detailed ...


2

If you go back to the sourced article in your graphics by Jennifer Johnson it has a more verbose colour legend. For the grey part it explicitly states very radioactive elements. Nothing left from stars. and her figure caption more verbosely Each element in this periodic table is color-coded by the relative contribution of nucleosynthesis sources, scaled ...


2

The presence of heavier elements makes the medium absorb more radiation. This means a nascent star that would have been able to collapse in the absence of metals, will lose its outer layers to radiation pressure as the core starts to heat up.


2

There is no simple answer to these questions - although I could be brief and say (i) No it doesn't and (ii) no they won't. If you make a simple two component atmosphere then the observed spectrum will be the flux-weighted combination of two spectra. $$ S_{\rm obs} = \frac{A_1 T_1^4 S_1 + A_2 T_2^4 S_2}{A_1 T_1^4 + A_2 T_2^4}\ , $$ where $A_1, T_1, S_1$ are ...


2

Learning how to tactfully and strategically read/skim/scan through many papers to find the bits of information or other papers that you're looking for (and leaving a more in-depth examination for later when you can invest the time, perhaps) is an essential ingredient in being able to answer one's own questions in research. Although I will not completely ...


1

The wind directions are not difficult to find. Just look at the dearest star to anyone of us, the Sun. It sets in the west and rises in the east. You could also look at the direction in which the stars move. This is the east-west (longitudinal) direction. When you notice the star that stays fixed in the sky you will know that it is Polaris. Just for double ...


1

You are comparing distributions in a way that they are not easily comparable and the eye is misled: watch the scaling of your axes of the plots you compare! In order to compare, you want to make sure that you use similar, either log-log for both graphs or linear-linear or something else - but identical in both graphs. Mind also that the size distribution of ...


1

Not being an expert in star formation, I found a well-written paper summary from which I conclude that typical star formation rates range between $6 \ldots 24 M_\odot / yr$. The blog quotes the following graph by M. Boquien, V. Buat, and V. Perret, see https://arxiv.org/abs/1409.5792 In this paper we investigate in isolation the impact of a variable star ...


1

Nickel 56 decays to Cobalt 56 via electron capture decay, with a half-life of 6.1 days and a decay constant of $\lambda = 1.31\times 10^{-6}$ s$^{-1}$. About 1.75 MeV of energy is lost as gamma rays and a further 0.41 MeV in the form of an electron neutrino (Nadyozhin 1994) Let's assume that we are talking about the period of time after the initial ...


1

This Okhubo 2009 paper presents two fiducial models: (a) stars between Pop III-1 stars of 40 to 300 M☉ stars not affected by stellar feedback which end in core-instability SN and BHs [they speculate that some became seeds of SMBHs]; and (b) Pop III-2 stars of 40 to 60 M☉ which do include radiative feedback and explode as Type II SNs, seeding the universe ...


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