# Tag Info

16

Possibly you are labouring under the misapprehension that the number of photons is somehow a conserved quantity? That isn't true, there are more photons at any given wavelength when you are deeper into the star, because there is a temperature gradient. Cooler material further out is less emissive because fewer atoms are in excited states. The temperature ...

7

You need to compare it with the spectrum of a similar galaxy at a known redshift, that would probably enable you to identify features with known rest wavelengths. If you can find such a template, then the best way of estimating a redshift for a galaxy spectrum like this, consisting of mostly weak and blended absorption features, is to cross-correlate your ...

6

You can probably get most if not all of your questions answered by perusing the main DESI web site, which I encourage you to check out. There is, for example, a nice video describing the assembly of the main focal plane elements (the fibers and the associated robot positioners) here. But in simple terms: the circular focal plane is divided into ten wedges (...

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Supplemental to @PeterErwin's answer, some more details on the five thousand "robots". Each fiber has a circular "patrol area" with a diameter of 12 millimeters, and these are located on a hexagonal array with a pitch (nearest neighbor distance) of 10.3 millimeters. Motion is implemented with eccentric axis (Θ–Φ) kinematics. Instead of x-y or r-Θ which use ...

5

I think you can understand that factor as follows: When photons lose energy, they're spread out over a larger wavelength range. Since there is a fixed number of photons, that means that the number of photons per observed wavelength bin decreases by a factor of (1+z), and hence the flux density, which is really what $f_\lambda$ is, decreases by this factor.

4

Yes, nebulae can often have very distinct colours. What produces those colours can depend on what elements are in the nebula material and what the temperature and density are. Generally speaking, green colours in a nebula are due to forbidden transitions in ionised Oxygen, though can feature the hydrogen $\beta$ Balmer line. Red colours can be due to ...

4

It depends what lines you are measuring and in what kind of star. When you measure the RV from a spectral line, you are measuring an intensity-weighted average RV over the region where the line is formed. For a star like the Sun, the photospheric lines are all formed within a layer no thicker than about a 1000 km and the differences in RV with depth in the ...

2

In this case itseems to mean that the depth of the line is 7 times its error bar below the continuum level. Impossible to answer. You say it can't be done, but the authors say that they fitted a Gaussian. You either use a rough estimate (attributable to Cayrel de Strobel 1988) of $$\Delta {\rm EW} \sim 1.5\frac{\sqrt{RP}}{{\rm SNR}},$$ where $R$ is the ...

2

For an authoritative source of data on exoplanets, your best bet is the NASA Exoplanet Archive. Their page for HD 219134b is here. For planet spectra, the archive has a page for planets with transit spectroscopy, but searching that page for HD 219134 yields no results. Note that transit spectroscopy is challenging, so there aren't very many planets to ...

2

It is difficult to say, there isn't a description in the paper as to how they arrive at those numbers. Given that the equivalent widths of the lines have signal to noise levels that mean they are only greater than zero at significance levels of 2-3, then it isn't clear these are meaningful numbers at all. However, here is how it might have been done. You fit ...

2

I know what an optically think/thick medium is... Okay so this isn't much more complicated. A medium or material can be optically dense or opaque at one wavelength, but fairly transparent at a different wavelength. If you look at the dark plastic window on a remote control for a TV or other appliance, you can't see through it. It's optically dense at ...

2

Disclaimer: this is speculative, since no images of a black hole have been taken with enough resolution in visible light. The black hole represented in the movie Interstellar is moderately realistic with the knowledge we have at present. In fact, the movie makers asked astrophysicists to contribute. From the article Gravitational lensing by spinning black ...

2

You are doing it incorrectly if you are trying to cross-disperse your Fig.2. You should be cross-dispersing your Fig.1. A "cross-dispersed" spectrum requires requires the dispersive elements to be at right angles. Your Fig.1 shows the dispersed spectrum from the echelle with overlapping orders. To separate the orders you view Fig.1 with the cross-...

2

You're asking two related but distinct questions here: Well, what about nearly-massive-enough-to-be-stellar objects? Why would there not be more of these - and the inevitable satellites involved in something of that mass - than there are solar systems? and also I would expect them to be in abundance, so, could this explain the dark matter mystery? For ...

2

Brown dwarfs (ie sub-stellar objects that are too small to support hydrogen fusion) do exist and some brown dwarfs have been found to host planets. An example of this is the brown dwarf 2M1207. It is a very dim object, about 25 Jupiter masses. It is still quite hot, so not completely "dark", and it hosts a planet with a mass of 3-10 Jupiters. ...

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This is not exclusive to spectroscopy applied to astronomy but general. Matter can interact with electromagnetic waves spanning a very wide range of frequency (energy). Also matter can emit electromagnetic radiation when in a kind of excited state. Due to the internal mechanism of absorption/emission it happens that the spectral characteristics can be ...

1

I would've posted this as a comment, but I lack sufficient reputation. I like your idea of fitting three Gaussians, but I think you should try fitting with one large positive Gaussian and two small negative Gaussians. My physical justification for this is that I would expect a lot of self-absorption from the H-alpha line, which should be well-modeled using ...

1

Visualizing a cross-dispersed echelle spectrum directly is a nice experiment to do (I do it with my upper-level astronomy students when I teach about this) but it's a little tricky because your intuition from other types of gratings can lead you astray in terms of how to arrange things for viewing. The key is to realize that the blaze angle of an echelle ...

1

While @planetmaker's comment is true if the lines come from the same source, you can have lines emerging from different physical processes which still appear to come from the same location. An example is absorption (or more rarely emission) lines from galactic winds, which are typically blueshifted with respect to the "systemic" redshift, i.e. the &...

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