You obtain a spectrum of an object in space. The spectrum consists of a number of sharp, bright emission lines. Is this object a cloud of hot gas, a cloud of cool gas, or a star?

In hot gases , the atoms keeps colliding with each other and sometimes the energy liberated during collision takes the electron to a higher level,thus, .The object is a cloud of hot gas and finally the electron returns back emitting photon

The object is a cloud of hot gas.

Explanation:

Kirchhoff’s laws establish that:

   • A solid, liquid or dense incandescent gas emits a continuous spectrum.  

   • A hot and diffuse gas produces bright spectral lines (emission lines).

   • A gas of lower temperature against a source of continuum spectrum, produces dark spectral lines (absorption lines) superposed in the continuum spectrum.

Stars are perfect examples for Kirchhoff’s laws. Since in the case of the stars, the photons that are received are not directly from the nucleus, but those that have traveled hundreds of thousands of years to reach the stellar atmosphere. Due to the stars are not at homogeneous temperature, density and pressure, but have gradients in different layers because of the nuclear reactions, superficial gravity or to their constant exchange of heat with their surroundings in an attempt to reach the thermodynamic equilibrium, the continuum observed in the stellar spectra comes from the inner layer of the photosphere, while absorption lines are formed in the outer layer of the photosphere and the stellar atmosphere. More accurately, a photon of the inner layer of the photosphere will be absorbed by an electron of an atom or ion that is in the outer layer, generating an electronic transition¹. The electron, upon returning to its base state will emit a photon or a series of photons that will not necessarily go in the same direction of the incident photon, creating an absorption line in the stellar spectrum.

On the other hand, in the case where the stars have surrounding material (diffuse gas), the atoms, molecules or ions in the medium are excited by the radiation that comes from the stellar atmosphere, thus producing an emission spectrum.

For this particular case, the object will be a cloud of hot gas, in which the source of continuum spectra is not in the same field of view that the cloud.

Key terms:

¹Electronic transition: When an electron passes from one energy level to another, either for the emission or absorption of a photon.

it is very well related to a lot of things but with my limited knowledge i can confidently say that the answer is space.