Narrative:

During descent on the STAR into ZZZ at about 12;000 MSL we got a TCAS intruder alert. It went yellow. From its position and altitude it was obvious to us that the other aircraft did not see us and was going to result in an RA. While getting ready for the RA we attempted to visually find the aircraft. While doing that; we got a level off RA. From a roughly 1;800 fpm descent we leveled off. I was finally able to see the aircraft as it was going underneath us. I don't know what it was but it was a small low wing GA aircraft with a full canopy and blue wingtips while the main body appeared to be all white. My first officer (first officer) said that it sounded like an rv but he didn't see it so couldn't say for sure. We reported it to ATC. They asked if we had to maneuver which we said yes. The first officer (first officer) told ATC it was a 172; but that was before I described it to him. ATC said thank you and asked no further questions and gave no further instructions. So we just rejoined the arrival and path and was able to hit all altitude gates. Flight landed without further incident. At no point did we receive a traffic callout from ATC. Without the RA; it is my opinion we would have collided with the intruder aircraft. Duty officer notified.we did what we needed to do and complied with the RA. We visually found the intruder to make sure that we avoided a collision. Only thing [is] that it would have been nice to get a traffic callout from ATC.

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Original NASA ASRS Text

Title: Air carrier Captain reported NMAC on descent.

Narrative: During descent on the STAR into ZZZ at about 12;000 MSL we got a TCAS intruder alert. It went yellow. From its position and altitude it was obvious to us that the other aircraft did not see us and was going to result in an RA. While getting ready for the RA we attempted to visually find the aircraft. While doing that; we got a level off RA. From a roughly 1;800 fpm descent we leveled off. I was finally able to see the aircraft as it was going underneath us. I don't know what it was but it was a small low wing GA aircraft with a full canopy and blue wingtips while the main body appeared to be all white. My FO (First Officer) said that it sounded like an RV but he didn't see it so couldn't say for sure. We reported it to ATC. They asked if we had to maneuver which we said yes. The FO (First Officer) told ATC it was a 172; but that was before I described it to him. ATC said thank you and asked no further questions and gave no further instructions. So we just rejoined the arrival and path and was able to hit all altitude gates. Flight landed without further incident. At no point did we receive a traffic callout from ATC. Without the RA; it is my opinion we would have collided with the intruder aircraft. Duty officer notified.We did what we needed to do and complied with the RA. We visually found the intruder to make sure that we avoided a collision. Only thing [is] that it would have been nice to get a traffic callout from ATC.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.