Question

What evidence do we have that spiral arms are regions of very active star formation? Give an answer that includes both nebulae and stars. What colors do we see?

In the context of galaxies and galaxy formation, what are tidal streams, and how are they created?

By carefully measuring the velocities of stars, astronomers can graph the rotation curves of a galaxy. What does the rotation curve of our galaxy tell us about the mass of our galaxy? What mysterious substance is most of the galaxy made of, and how do we know this?

Answer 1

Spiral galaxies are galaxies composed of flattened star forming dark component which have spiralbarms. Spiral nebulae was the historical name of spiral arms. Spirak arms are parts of spiral patterns of our gallexy. Star formation occurs in spiral arms asbthe gas clouds are compressed in arms to form star.

There are many theories , densitybwave theory states that spiral structure isba wave that moves through thebdisk causing stars and gas to clump up along the wave. The stars pile up at the spiral arm as they orbit the centre.Clouds and stars speed up to density wave and arebtugged backwards as they leave. Clouds compress and form stars.

Another theory says that when supernova shock wave reaches the gas cloud , it compresses and stimulate the formation of stars. Some produce supernova explotion to keep it going.

Many research and data suggest tgat a giant molecular cloud was compressed by spiral arm which may have triggered star formation.

Spiral arms in gallaxies are blue in colour because mostly short lived massive stars are formed here which die shorlty after the increased wavefront move.Nebulae that scatter light are blue.

Tidal stream

When dwarf galaxies xomes very close to larger galaxies tidal distruption occurs. The gravitational force between one side of galaxy and other side rip the star from the larger galaxy. They orbit the larger galaxy. This produces tidal stream. They give evidence to the vilolent merging history of galaxies.

Rotational curve

Rotational curves tell us that the gallaxy is much bigger than what is expected and that the luninous part is just the tip of an iceberg! Most of the mass of galaxy exist in the form of dark matter.. Rotational curve measurements of spiral galaxies is one of the most compelling set of evidence for dark matter. The surface luminosity of disk falla off exponentially with radius  ,implying that the mass if luminous matter is concentrated at centre. The flattening of raotational curve suggest thatbthe total mass of galaxy within some radiys R is increasinf always with increase in R.Thisncan be explained if there is a large amountbof invisible graviting mass. The missing is 10ntimes the luninous mass. Mostbof dark matter must be in the halo of galaxy. The rotational curve data is consistant with dark matter distributed in their halo. This dark halo comstitues much of galaxies total mass