Why no dark lines for black body?

Question

Black body is an object, hence it's made of atoms. Depending on what atom it consists of, that's how the emitted spectrum should be in my opinion. If it contains hydrogen, absorbed light on black body would excite electrons and electrons of hydrogen can only produce certain wavelengths. Hence, there must be dark lines in the final, emitted radiation(which is called - black body radiation).

how does perfect black body have continues spectrum (with no dark lines ?) That means when we talk about perfect black body, we shouldn't talk about atoms inside it as there's no atom existing that absorbs or emits light at any wavelength.

Answer 1

Black body is an object, hence it's made of atoms.

No, a black body is not made of atoms. It is an imaginary object which has the special (but physically unrealizable) property that it is a perfect absorber - any incident electromagnetic radiation is absorbed, with zero reflection and zero transmission. Under certain conditions, many objects in nature (stars, rocks, people) can be modeled as black bodies (or some modification thereof) to varying degrees of accuracy.

Depending on what atom it consists of, that's how the emitted spectrum should be in my opinion. If it contains hydrogen, absorbed light on black body would excite electrons and electrons of hydrogen can only produce certain wavelengths.

The electromagnetic radiation emitted by an object is not determined exclusively by the electronic transitions which are possible in its constituent atoms. Solid objects have electronic energy levels which form broad, continuous bands, not discrete spectral lines. Stars are comprised of plasma, not neutral atoms; photons are produced via the scattering of charged particles (bremsstrahlung) and their energies are further randomized via Compton scattering.

That means when we talk about perfect black body, we shouldn't talk about atoms inside it as there's no atom existing that absorbs or emits light at any wavelength.

Yes, that's right.

Answer 2

A blackbody must be capable of absorbing (and emitting) light at all wavelengths. Anything in nature that approximates to an ideal blackbody (a tiny hole into a large cavity) has this property.

You can find out what processes lead to this absorption in the linked question.

What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

In your example of a hydrogen gas (and I guess to are talking about stars), the "continuum absorption" in the visible part of the spectrum is provided by photoionisation of the the H$^{-}$ ion, which has an ionisation energy of just 0.7 eV and absorbs photons across the visible part of the spectrum. This means that stars like the Sun have spectra approximating to blackbodies in the visible part of the spectrum.

The Sun does of course have "dark lines" (Fraunhofer lines) in its spectrum. This is because both: (a) absorption is enhanced at the wavelengths of atomic transitions as you suggest; and (b) there is a temperature gradient in the Sun which then means we see to depths occupied by cooler plasma at those wavelengths. Thus the lines are not totally dark, they are just less bright than the surrounding continuum. The non-isothermality of the Sun's atmosphere is why it doesn't emit an ideal blackbody spectrum.