# What color is a neutron star?

#### Forestman

If a person could orbit close to a neutron star what color would it appear to be?

I have always imgained them to be blue, but this might be totally wrong. :uhh:

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#### gtring

As I understand it, at the surface of a neutron star, most light is emitted in the X-ray range. In the visible range, red is emitted at about the same as blue and the other colors, so it would appear white to human eyes.

Your human eyes and body should not orbit too closely, or else tidal effects will turn you into spaghetti.

Thanks gtring

#### fasterthanjoao

Your human eyes and body should not orbit too closely, or else tidal effects will turn you into spaghetti.

spaghettification!

#### Chronos

Gold Member
Most neutron star radiation we detect is in the x ray spectrum, implying they are fantastically hot. It is, however, unlikely much if any of this radiation is directly emitted by the neutron star. Black holes also 'emit' high energy radiation but obviously none of it is emitted by the black hole itself. It is the result of matter collisions in the process of being devoured by the black hole. Most neuton star radiation can probably be attributed to this same effect.

#### stevebd1

Gold Member
A couple of equations that you might find of interest (based on a static 2 sol mass NS with a radius of 12 km)-

Tidal force equation (m/s2/m)-

$$dg=\frac{2Gm}{r^3}dr$$

The 'ouch' radius (which is derived from the tidal force equation)-

$$r=\sqrt[3]{\frac{2Gm}{dg}dr}$$

based on tidal forces being equivalent to 1 Earth g from head to foot (dg=9.82, dr=2), you're getting into within a radius of ~4,800 km of a neutron star. Based on a maximum limit of dg=10 Earth g from head to foot, ~2,300 km (though by this point you've probably passed out).

Based on an orbit of 4,800 km, a neutron star with a 12 km radius would appear to be half the size of our sun as it appears to us from Earth (or equivalent to a 5 mm disk held at arms length- 2r/d, d=distance).

Gravitational redshift-

$$z=\left(1-\frac{2M}{r}\right)^{-1/2}-1$$

where r is the radius of the star, M is the gravitational radius (M=Gm/c2) and z is the fractional shift in the spectral wavelength

$$\lambda_o=(z\cdot \lambda_e)+\lambda_e$$

where λo is the observed wavelength and λe is the emitted wavelength

as gtring has already stated, blue light (475 nm) emitted from the surface of the NS would appear red (667 nm)

If the neutron star being approached was a http://en.wikipedia.org/wiki/Magnetar" [Broken] to contend with.

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#### Forestman

Thanks stevebd1, and thanks for putting links to the magnetar and diamagnetism. That was awesome about that frog being levitated. I have learned so much since I have been on this site!

#### Dmitry67

it would appear white to human eyes.
Violet.
Those who witnesses nuclear explosions described the color of explosion (millions K) as violet

#### Chronos

Gold Member
stevbd1 put the math to the fire,, well done! I only object to the mass equivalence thing. Volume to mass ratio is not linear.

#### stevebd1

Gold Member
I only object to the mass equivalence thing. Volume to mass ratio is not linear.
Hi Chronos

I assume your saying the star would appear smaller due to curved space (i.e. the coordinate radius of the star would be less than 12 km) or are you talking about something else?

#### Creator

If the neutron star being approached was a http://en.wikipedia.org/wiki/Magnetar" [Broken] to contend with.
Betcha the gammas get ya first.

Good post...

" It has even been said that at a distance halfway to the moon, a magnetar could strip information from a credit card on Earth."

I've been wondering how they stole my Discover card info.
??
...

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#### Chronos

Gold Member
Mostly correct, stevebd1, neutron stars do not radiate to any appreciable extent. They do strip matter from any convenient source.

Staff Emeritus
It's true that neutron stars don't radiate much, but that's because they are small. However, they are quite hot, which means they would be very bright if you got close enough to them.

#### JeffKoch

Violet.
Those who witnesses nuclear explosions described the color of explosion (millions K) as violet
Interesting, I hadn't ever heard this. Do you have links to this observation?

#### QuantumPion

Gold Member
I'm just curious since I never thought about this before. If you had a lump of neutrons the size of a baseball at room temperature, what would it look like? Black? Transparent? Metallic? I know that photons do interact with neutrons but I'm not sure what the macroscopic effect would be.

#### stevebd1

Gold Member
Neutrons are only bound together in a neutron star due to the massive gravity. The smallest neutron star predicted is ~1.35 sol mass so if you were to take a baseball size 'chunk' of neutron degenerate matter, it would fly apart in a burst of energy due to massive unconfined pressure (which is normally overcome by extreme gravity).

Slightly off topic but strange matter (which is a quark-gluon plasma type composed of up, down and strange quarks) on the other hand is theoretically more stable than nuclear matter (i.e. iron), http://arxiv.org/PS_cache/astro-ph/pdf/0407/0407155v2.pdf" [Broken] (page 19, fig 11) so small pockets of strange matter might exist without the need for gravity (keeping in mind that 1 cm cubed of strange matter would weight anywhere upwards of 2 billion tonnes.

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#### Bob S

Neutrons are only bound together in a neutron star due to the massive gravity. The smallest neutron star predicted is ~1.35 sol mass so if you were to take a baseball size 'chunk' of neutron degenerate matter, it would fly apart in a burst of energy due to massive unconfined pressure (which is normally overcome by extreme gravity). .
Free neutrons are radioactive, and have a half life of about 886 seconds. Unless phase space prevents it, they will turn into a proton + electron + neutrino. It is not clear that phase space would prevent this in a neutron star. So how long will a neutron star last?

Staff Emeritus
It is not clear that phase space would prevent this in a neutron star.
Of course it is. Just because it's not clear to you doesn't mean it's not clear to anyone.

In a neutron star, you have available energy levels for about 10% as many electrons as you have neutrons. Once these fill up (and they are filled with original electrons from the star as soon as the neutron star forms) you can only add an electron by giving it enough energy to be in an unoccupied and high energy state. This additional energy kinematically blocks neutron decay, making such stars stable.

#### Chronos

Gold Member
This [what vanadium noted] also leads to the deduction neutron stars have an iron crust. Degenerate matter is pretty weird stuff.

#### Christof

I'm a new member and I hope this is not too far off topic: Would frame dragging in any way alter the appearance of a NS? (visibility, color, etc.)

#### Chronos

Gold Member
Frame dragging effects would be negligible at the surface of a neutron star.

Thx Chronos.

#### stevebd1

Gold Member
I'm a new member and I hope this is not too far off topic: Would frame dragging in any way alter the appearance of a NS? (visibility, color, etc.)
To take into account frame dragging, the redshift as observed from infinity would be-

$$\alpha=\frac{\rho}{\Sigma}\sqrt{\Delta}$$

where

$$\rho=\sqrt{r^2+a^2 \cos^2\theta}$$
$$\Sigma=\sqrt{(r^2+a^2)^2-a^2\Delta\ \sin^2\theta}$$
$$\Delta= r^{2}+a^{2}-2Mr$$

and M=Gm/c2, r in this case would be the radius of the neutron star (say approx 10-12km) and m is the mass (say 2-2.2 sol)

(Note: while for a black hole, $a$ can be anything up to $a=M$, for a spinning neutron star, the max is more likely around $a=0.4M$ otherwise shredding would occur)

$$z=1/\alpha -1$$

which can be rewritten-

$$z=\Sigma\left(\rho\sqrt{\Delta}\right)^{-1}-1$$

and as in post #6-

$$\lambda_o=(z\cdot \lambda_e)+\lambda_e$$

where λo is the observed wavelength and λe is the emitted wavelength.

#### lpetrich

Returning to the OP's question, let's consider blackbody colors, since these will be the colors of anything with a thermalized spectrum. To see what, check on sites like What color is a blackbody? - some pixel rgb values.

Anything above 6700 K will look bluish, and above 17500 K or so, the colors don't change much. I like to call the color in this limit "Rayleigh-Jeans blue", because one is seeing the Rayleigh-Jeans limit of the blackbody spectrum.

The hotter stars all look Rayleigh-Jeans blue, and all the temperature values and estimates I've found for pulsars are greater than 10,000 K, so they also will look Rayleigh-Jeans blue.

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